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"Young adolescents and black women should strive for gains at the upper end of the recommended range. Short women ( < 57 cm, or 62in) should strive for gains at the lower end of the range. bBMI is calculated using metric units.

The recommended target weight gain for obese women (BMI > 29.0) is at least 6.0 kg (15 lb).

Reproduced by permission of the National Academy of Sciences (National Academy Press).

"Young adolescents and black women should strive for gains at the upper end of the recommended range. Short women ( < 57 cm, or 62in) should strive for gains at the lower end of the range. bBMI is calculated using metric units.

The recommended target weight gain for obese women (BMI > 29.0) is at least 6.0 kg (15 lb).

Reproduced by permission of the National Academy of Sciences (National Academy Press).

objective), and do more to contribute to obesity in the mother (59,60,73). Further evidence to support this view is now emerging: To and Cheung (64) showed that gestational gains greater than two standard deviations above the mean were not associated with any significant increase in birthweight, but that they were associated with higher postpar-tum weight retention. However, the results of studies which measure body weight too soon after delivery (53,57,60,64) should be interpreted with caution as they do not give women with high gesta-tional gains enough time for their body weight to equilibrate after delivery (12). In this way, they can erroneously conclude that high weight gains during pregnancy predispose women to long-term weight gain. However, there are numerous studies which do give women sufficient time for body weight to stabilize following pregnancy (49,50,54,57,61-63), and all of these suggest that women with high gesta-tional gains are at increased risk of long-term weight gain.

Greene et al. (49) investigated the effect of weight gain during pregnancy on maternal body weight in a group of 7116 US women. All women in this study had experienced more than one pregnancy so that their change in body weight from one pregnancy to the next could be examined. Inter-pregnancy weight gain was defined as pre-pregnant weight in the second study pregnancy minus pre-pregnant weight in the first study pregnancy. This measure of long-term weight gain was then compared to the gesta-tional weight gains that the women experienced in their first study pregnancy. Figure 20.5 shows the mean gestational weight gain for 11 maternal weight gain groups plotted against the mean inter-pregnancy weight change. It is clear from this figure that women with high gestational weight gains are at greater risk of persistent weight gain when compared to women with relatively lower gestational weight gains. The authors concluded that gesta-tional weight gains in excess of 9.1 kg were positively related to the amount of weight retained postpar-tum, and that: 'the more weight a woman gains, the more she retains'.

Similar results have been obtained in more recent studies. For example, Thorsdottir and Birgisdottir (63), in their study of women of normal BMI before pregnancy, showed that women with high gesta-tional weight gains (18-24 kg) were, on average, 2.6 kg heavier (more than a year) after pregnancy than they were before, whereas women with comparatively moderate gestational gains (9-15 kg) weighed less after pregnancy than they did before. The almost universal observation that high gesta-tional gain is positively associated with persistent weight gain is particularly disheartening since most women today experience gestational gains well in excess of 9.1 kg (47,50,54), and are indeed recommended to do so (6) (Table 20.3). It therefore appears that pregnancy is associated with a persistent increase in body weight simply because it is a period of positive energy balance during which some women gain excessive weight.

Smoking Cessation

Another group of women who are at increased risk of persistent weight gains following pregnancy are those who quit smoking (54). Statistics show that 38% of women in the UK are classified as smokers at conception and that 26% of these women will quit smoking during pregnancy (74). Ohlin and Rossner (54) showed that women who gave up smoking early in pregnancy gained significantly more weight during pregnancy than other groups. Women who did not start smoking again during the follow-up year experienced significantly greater long-term weight gains (3.4 kg) than did continuous smokers (0.9 kg) and non-smokers (1.5 kg; P < 0.01), while women who resumed smoking within 6 months of delivery had long-term weight gains similar to those of non-smokers (1.4 kg). Smoking is known to increase basal metabolic rate by around 10% (75), and for this reason women who smoke

Figure 20.5 Mean weight change by mean weight gain in pregnancy for 11 prenatal weight gain groups, after adjusting for the effects of inter-pregnancy interval, smoking, gestation at registration, socioeconomic status, breast-feeding at hospital, complications of pregnancy, gravidity, maternal age, per cent ideal body weight, marital status, and race. From Greene et al. (49). Reprinted with permission from the American College of Obstetricians and Gynecologists (Obstetrics and Gynecology 1988; 71: 701—707)

Figure 20.5 Mean weight change by mean weight gain in pregnancy for 11 prenatal weight gain groups, after adjusting for the effects of inter-pregnancy interval, smoking, gestation at registration, socioeconomic status, breast-feeding at hospital, complications of pregnancy, gravidity, maternal age, per cent ideal body weight, marital status, and race. From Greene et al. (49). Reprinted with permission from the American College of Obstetricians and Gynecologists (Obstetrics and Gynecology 1988; 71: 701—707)

throughout pregnancy tend to have lower long-term weight gains than those who do not (49,54,56). For similar reasons, women who quit smoking during pregnancy are at greater risk of subsequent weight gain. While cigarette smoking is protective against weight retention postpartum, the observed benefit of smoking does not offset the toxic effects of cigarettes on the health of both the mother and her child (71).

Risk Factors Common to Both the Pregnancy and Postpartum Periods

Changes in Activity

Until recently (55,56,61,65), the potential effects of changes in activity on maternal body weight had received relatively little attention. Schauberger et al. (56) found that mothers who resumed work within 2 weeks of delivery retained significantly less weight by 6 months postpartum than did mothers who went back to work later. However, this relationship did not seem to be the result of differences in activity between working mothers and those who stayed at home, as there was no evidence of an association between activity and long-term weight gain. Ohlin and Rossner (55) also found no difference in long-

term weight gain between mothers with altered leisure activities following pregnancy, and neither they nor Harris et al. (65) found any association between returning to work and long-term maternal weight gain. Harris et al. (65) did, however, find long-term weight gains to be significantly greater (3.1 kg greater) among women who reported undertaking less exercise after pregnancy than they did before pregnancy. In this descriptive study, lack of opportunity, energy, time and money were commonly cited reasons for declines in activity postpartum.

One explanation for these equivocal findings is that the intensity of activity, and/or the nature of employment, determines the absolute importance of changes in activity and/or employment status for long-term weight gain. Alternatively, changes in energy intake might be more important than changes in activity patterns in the development of obesity among women with predominantly sedentary lifestyles, such as those in Western Europe. As with all reported behaviours, care must be taken when evaluating the possible influence of changes in activity on body weight after pregnancy. Reports of activity levels may be embellished pictures of reality, and when information on pre-pregnancy activity patterns are gathered retrospectively they may be subject to recall bias. In particular, retrospective recall of certain behaviours may often be biased if mothers misreport these behaviours as post-hoc justification for any weight gains they may have experienced. Likewise, the complex interrelationships between many behavioural characteristics (68) indicates the need for caution when evaluating the results of descriptive studies, since any relationships observed may simply be the result of confounding. For example, the relationship between reduced activity and long-term weight gain that was found by Harris et al. (65) in their descriptive study was not statistically significant after adjustment for the confounding effects of other factors including changes in: energy intake, access to food and body image, as well as parental obesity, social support, parity, maternal age, marital status, smoking status, gesta-tional weight gain and pre-pregnant weight (68).

Changes in Energy Intake and Patterns of Eating

It has been shown that changes in dietary intake are a common characteristic of pregnancy and the post-

partum period (50,55,76,77). A large proportion of new mothers stay at home for a time after birth, and whilst at home they often have greater access to food throughout more of the day than they had before they were pregnant (65,66). Among mothers participating in the Stockholm Pregnancy and Weight Development Study, those who displayed less well-structured eating patterns retained more weight by 1 year postpartum (55). Women who increased in body weight also reported that their energy intake had risen during pregnancy and the postpartum period because they ate larger portions and snacked more frequently. These women also skipped breakfast and lunch more regularly than women who put on less weight (55).

In a descriptive study by Harris et al. (65), more than 70% of mothers felt they ate differently from the way they did before they were pregnant. Mothers who felt they ate more after their children were born displayed significantly greater long-term weight gains (2.78 kg) than those who felt they had not increased the amount they ate ( — 1.15 kg). Although this association just failed to reach significance after accounting for the effects of confounding factors (P = 0.08; (68), this finding may be important since more than a fifth of the mothers in the group reported increasing their energy intake after the birth of their child. In this study, more than 13% of the mothers who reported increasing their energy intake were newly classified as overweight or obese (BMI > 26.0) after pregnancy (68). Mothers often report having to grab their meals when they can, and devoting more time to feeding their children than they do to preparing food for themselves (65). It is therefore possible that these changes in maternal eating habits and patterns of eating predispose some women to gain weight after pregnancy. It is important that women are aware of this possibility, so they can take steps to prevent these new behaviours from tracking into the future.

Dieting and Attitudes to Weight Gain

Studies show that between 13 and 52% of European women undertake some form of dieting during the first year after birth (54,65). There is, however, little evidence to suggest that these women are any more successful in returning to their pre-pregnant weight than are those who do not diet during the postpartum period (65). For some women, a previous history of dieting has been shown to be positive ly related to pregnancy-related weight gain (54) but this is not the case for all women (65). It is likely that the impact of dieting on body weight is just as difficult to predict after pregnancy as it is at any time in a woman's life, although some believe the immediate post-pregnancy period to be a better-than-average time to achieve a successful persistent weight loss (16,78,79). Some women do, however, appear to experience marked changes in their attitudes to weight gain both during and after pregnancy. For woman who normally restrain their eating to preserve their figures, the inevitable change of shape during pregnancy may serve as justification for 'letting themselves go'. Others believe they should 'eat for two' or fear that by depriving themselves they are depriving their child. In a recent UK study more than 40% of mothers felt it was inevitable that they retained some of the weight that they gained during pregnancy (65). In this study changes in attitude to weight gain were also assessed using the 'drive for thinness' sub-scale of the Eating Disorder Inventory (80). This scale contains seven questions that assess 'concern with dieting, preoccupation with weight, and entrenchment in an extreme pursuit of thinness'. Harris et al. (65) showed that mothers displayed significantly greater scores in their drive for thinness after pregnancy than they did before (P < 0.001). Interestingly, those mothers with an increased drive for thinness had not gained significantly more weight than those mothers who displayed no increase in drive for thinness after pregnancy. Such changes in attitudes to weight gain, which are independent of actual changes in body weight, are suggestive of an increased vulnerability to eating psychopathology during the post-partum period (81). For these women, pregnancy may represent an important 'risk period' for the development of obesity.

Change of Body Image

In Western society, the notion of beauty in women equates with being thin (82), and it is therefore not surprising that women are commonly concerned about their weight and shape both during and after pregnancy. Younger or primiparous women can be somewhat unrealistic in forecasting their expected postnatal weight, and the greater the difference between anticipated and actual postnatal weight, the greater the postnatal weight dissatisfaction (83). This may be important as others have found that, after accounting for the effects of potential confoun-ders and known risk factors for maternal obesity, women who were more dissatisfied with their bodies after pregnancy were significantly more likely to have higher long-term weight gains than those who displayed no increase in dissatisfaction with their bodies after pregnancy (P = 0.01; (68)). This association suggests that either (i) increased body image dissatisfaction predisposes mothers to gain more weight in association with pregnancy or (ii) mothers who gain more weight following pregnancy are more dissatisfied with their bodies. Bi-variate correlation analyses have shown mothers who were more dissatisfied with their bodies post-partum to be more likely to report an increase in energy intake following pregnancy. While it is impossible to determine the causal direction of these relationships, previous analyses have demonstrated significantly higher levels of depression among women with increased body dissatisfaction following pregnancy (68). The changes in body image that occur following pregnancy might therefore lead to depression and reduced self-esteem, which in themselves are known risk factors for increased energy intake (84), particularly among women (85,86).

Risk Factors Associated with the Postpartum Period

Breast-feeding

Breast-feeding has traditionally been thought to facilitate weight loss. In clinical practice, women are often instructed to try to breast-feed as much as possible to revert to normal weight after delivery (72). This seems logical since a full lactation requires about 500 kcal a day, which for many women in this age group may constitute around 20 to 25% of their daily energy requirement (72). In theory, the metabolic costs of lactation can be met in four main ways: women can either (1) increase their energy intake, (2) mobilize their energy stores, (3) increase their metabolic efficiency, (4) reduce their energy expenditure, or employ a combination of these four adaptations. From a nutritional point of view, it seems reasonable to assume that the adipose tissue that is stored during pregnancy provides energy for the child during the lactation period. However, it is evident from the literature that postpartum weight changes in breast-feeding women are highly vari able both within and across populations (87).

Several studies have supported a positive influence of breast-feeding on weight loss (62,88,89), while others have shown a negative influence (26,46,90) or little influence at all (50,56,58,60,63,69,90-92). Overall, breast-feeding appears to have its greatest effect in the early months (54,62,87,89,91,93), yet by 12 months post-partum (after which time most women have stopped breast-feeding) the difference in weight loss between those who had been breast-feeding and those who had not is minimal (54,92,93). Only in the small numbers of women who breast-feed for lengthy periods (around 12 months), does breast-feeding appear to be related to increased weight loss (54,93).

In the Stockholm Pregnancy and Weight Development Study, Ohlin and Rossner (54) found breast-feeding to have only a minor influence on postpartum weight development: In this study a scoring system was constructed to reflect both the duration and intensity of breast-feeding. Their system gave every month of full lactation a score of 4 points, and every month with mixed feeding a score of 2 points. This score was then multiplied by the number of months during which women indicated that they breast-fed. This system made it possible to sum up periods of complete lactation with ensuing periods of partial lactation, and in this way a range from 10 to 48 points was obtained to give a rough estimate of the total energy expenditure for milk production. Although they found a significant relationship between lactation score and weight retention, the relationship was very weak (r = — 0.09, P < 0.01). However, they were able to demonstrate that women with high lactation scores lost more weight during the first 6 months following delivery, but by the end of the year the difference between the groups was limited (Figure 20.6).

Insensitive definitions which fail to reflect the duration and intensity of breast-feeding are inevitably responsible for some of the conflicting results on the influence of breast-feeding on body weight. Likewise the paucity of data on energy intakes makes interpretation and inter-study comparisons difficult. Nevertheless, in most reports, rates of weight loss do not differ between breast-feeding and non-breast-feeding women, and only subtle short-term differences in body composition are observed following pregnancy (87). In those studies that demonstrate a statistically significant effect of breastfeeding on body weight, the contribution to the

Figure 20.6 Weight loss (kg) from 2.5 months postpartum in groups with different lactation scores. * Differed from 0-9,10-19 (P < 0.05). ** Differed from 0-9, 10-19 (P < 0.01). From Ohlin and Rossner (54). Reproduced by permission of MacMillan Press Ltd

overall variability in postpartum weight change is minor (87). In developed countries, it seems that changes in eating behaviour and lifestyle affect women in such a way that they use their adipose tissue storage only when food is not readily available. For example, several studies (94,95) indicate that well-nourished women with foods freely available do not necessarily mobilize body fat during lactation, but rather tend to cover the energy costs of lactation by an increased energy intake and possibly by decreased physical activity (96). In this way changes in eating behaviour patterns after delivery seem to counteract the inherent weight controlling potential of breast-feeding. This behaviour may go some way to explain why women from developed countries tend towards obesity following pregnancy, while women in developing countries tend towards maternal depletion. In developed countries, it is likely that the effects of breast-feeding on body weight are sufficiently limited to warrant minimal emphasis on breast-feeding as a means of minimizing postpartum weight retention.

Psychosocial Factors: Depression, Self-esteem, Stress and Social Support

There are a number of other psychosocial factors, such as depression, self-esteem, stress and social support, that might also influence maternal energy balance after pregnancy. These risk factors have received relatively little attention and are notoriously difficult to measure. As such, absence of an association between any of these variables and weight gain does not always mean that no association exists. These factors rarely operate in isolation and usually form part of a complex milieu of social and emotional factors that make up a woman's life. Even when psychosocial variables, like these, are found to be related to a biological outcome, like body weight, questions necessarily arise about the processes that underlie the relationships. If data on psychosocial symptoms and postpartum weight are collected simultaneously, the direction of influence of any relationship is usually unclear. For example, women may experience depressive symptoms as a result of their weight or depressive symptoms may modify biological or behavioural processes that affect body weight. Therefore, the direction of these causal relationships is usually difficult to predict and most studies are only able to describe tentative associations between psychosocial variables and body weight. For these and other reasons, studies investigating the influence of psychosocial factors on body weight are scarce.

Some researchers have investigated the effects of stress on changes in body weight following pregnancy (65,67). However, these studies have found no association between life event stress or the stresses of parenting and long-term weight gain following pregnancy (65,67). However, both Walker (67) and Harris et al. (65) found high levels of stress and low levels of social support to be related to higher depressive symptoms, which in themselves have been shown to be related to long-term weight gain following pregnancy (67,83). Others have suggested that weight loss following delivery might depend upon the amount of social support each mother receives (97) and that women with little social support, who might feel isolated and lonely as a result, respond to their lack of support by compensatory

'comfort' eating. There is tentative evidence to support this view (67,68), although the mechanisms that underlie this association are far from clear. Other researchers have explored the effects of changes in self-esteem following pregnancy. Given the importance of body weight to perceived attractiveness, and the importance of attractiveness to a women's self-image (82), it follows that a woman's satisfaction with her weight is likely to be a central aspect of her self-esteem. In a study by Walker (67), mothers were asked at 1 year postpartum whether their current body weight affected how they felt about themselves, and just less than half of the entire sample (47%) reported that their self-esteem had decreased. Reduced self-esteem and the normally inescapable demands imposed by motherhood can set the stage for depressive symptoms during the period after childbirth (67). In fact in Walker's study (67), those mothers with gains at 1 year postpartum of at least 5 kg reported high depressive symptoms more often than women with lesser gains (53% vs. 28%).

It is likely that psychosocial factors play an important role in the development of maternal body weight following pregnancy. However, well-designed prospective studies are needed if the effects of these factors are to be elucidated.

Pre-existing Risk Factors

Pre-pregnant Weight

Along with gestational weight gain, pre-pregnancy weight is the other variable that consistently shows a significant positive association with pregnancy-related weight gain (19,49,50,56,58). This suggests that women who enter pregnancy with an already high body weight are at greater risk of long-term weight gain than are women of lower body weight. Furthermore, the weight development of women who begin pregnancy overweight is known to be more variable (52,54,57,66). For example, Keppel and Taffel (57) showed that among normal weight women (BMI 19.8-26.0 kg/m2) whose gestational gains were within the Institute of Medicine's recommended range (see Table 20.3), 20% retained 4 kg or more, while 29% lost weight by 10-18 months postpartum. In contrast, of the overweight women (BMI 26.1-29.0 kg/m2) who gained as recommended, 38% retained 4kg or more, while 33% lost weight.

This variability of pregnancy-related weight gains is a remarkably consistent feature of postpartum weight change in overweight women.

To some extent, the higher weight gains of heavier mothers may be the result of fundamental differences in physiology and/or lifestyle that place these women at increased risk of gaining more weight, irrespective of pregnancy. Differences in nutritional status (BMI) are the consequence of differences in physiological characteristics and/or lifestyles that cause some women to gain more weight than others. There are, however, two important points to consider. Firstly, overweight women are known to under-report their body weight to a greater extent than women of lower weight (18,19). Because weight retention is usually calculated as: body weight measured after pregnancy minus pre-pregnant weight, overweight women may appear to be retaining more weight than lighter women with the same weight retention, when calculations are based on self-reported pre-pregnant weight (50,58). This does not, however, explain the greater weight gains observed among overweight women in studies that rely on pre-pregnant body weights that are measured in early pregnancy (50,56). Secondly, the increased risk of weight retention observed among overweight women might simply be an artefact of longitudinal study design: for example, we might observe this relationship simply because heavier women gain more weight over a fixed period of time than lighter women, regardless of pregnancy. This would give the impression that overweight women are at greater risk of pregnancy-related weight gains, when in fact they are simply at greater risk of weight gain generally (98). With this in mind, there is little empirical evidence to suggest that overweight women are at any increased risk of maternal obesity when compared to women of lower body weight (98).

Heredity

Few studies have considered the effects of heredity on postpartum weight, and this represents an important oversight since heredity is considered to be one of the three most important factors determining body weight (99). More than 50 years ago clinicians noticed that mothers of the maternal obese had more often suffered from obesity after pregnancy than had mothers of women whose obesity was not pregnancy-related (14,100). These early observa-

tions suggested that a history of maternal obesity in a woman's mother might be a risk factor for pregnancy-related weight gain.

Harris et al. (68) assessed the relative importance of heritable characteristics and lifestyle in the development of body weight following pregnancy in a group of mothers from south-east London. In this study, 74 mothers of low antenatal risk who had been weighed during the first trimester of pregnancy were followed up 2.5 years after delivery. 'Heritable' predisposition to gain weight was assessed using the Silhouette Technique (101). This technique asks subjects to score the degree of obesity in their parents using a series of nine silhouette drawings showing bodies of increasing obesity, ranging from very thin to very obese (numbered in order from 1 to 9). This technique has been validated by correlation with BMI, as offspring's selected silhouettes of their mothers have been shown to correlate well with measured maternal BMI (r = 0.74: Sorenson and Stunkard (101)). After adjusting for the effects of potential confounders and known risk factors for maternal obesity, women who selected larger silhouettes to represent their biological mothers were significantly more likely to have higher long-term weight gains than those who selected thinner maternal silhouettes (r2 = 0.083, P = 0.004; see Figure 20.7). Interestingly, long-term weight gain was not associated with the size of the biological father (P = 0.50). This shows that a 'heritable' predisposition to gain weight is independently associated with long-term weight gain following pregnancy, and suggests that some component of heredity might determine why some women gain more weight than others in association with pregnancy. A number of previous studies have shown that the familial resemblance of obesity has a genetic component which may be inherited (101-104), with twin and adoption studies indicating that genes play a major role (102). However, it is also possible that offspring 'inherit' lifestyles that predispose them to gain weight by adopting similar eating habits and exercise patterns to those of their parents. The absence of an association between long-term maternal weight gain and the size of the biological father (68) suggests that inherited maternal attitudes to body weight and weight gain, as well as postpartum lifestyle, might be more important than any genetic characteristic inherited from either parent. Nevertheless, it is likely that both processes have a role.

Figure 20.7 Mean long-term weight gains of 74 women according to the number of silhouette selected to represent their biological mother. Error bars show standard error about the mean and sample size is shown in parentheses. From Harris et al. (68). Reproduced with permission of the BMJ Publishing Group

Figure 20.7 Mean long-term weight gains of 74 women according to the number of silhouette selected to represent their biological mother. Error bars show standard error about the mean and sample size is shown in parentheses. From Harris et al. (68). Reproduced with permission of the BMJ Publishing Group

Maternal Age

The results of studies on the modification of pregnancy-related weight gain by age are limited and conflicting. Some show no significant association between maternal age and weight gain following pregnancy (49,50,70), while others show a significant negative influence (54,62,63). Another recent study showed that among white women, younger compared to older women were found to be more likely to experience substantial pregnancy-related weight gains (defined as gains > 11.4 kg during the 10-year study period), while among black women, the opposite was true (105).

It seems logical to assume that older women might be at greater risk of pregnancy-related weight gain, as a direct consequence of the reduced metabolic efficiency that accompanies advancing age. Older mothers may also be less concerned about slimness than younger women (63). Janney et al. (62) showed that age rather than parity influenced the rate of postpartum weight retention after a first or second pregnancy. In this study, older women were shown to be significantly less likely to return to their pre-pregnant weight than younger women. They also had slower rates of weight loss than their younger counterparts. The significant interaction between maternal age and time since delivery is illustrated in Figure 20.8 (62). This model suggests that for women who experience pregnancy between the ages of 20 and 35 years, regaining pre-pregnant weight would be anticipated, but for women who

Figure 20.8 Predicted weight retention curves for women of various ages (hypothetical married women who fully breast-fed for 6 months and gained 15.9 kg during pregnancy). •, 40 years; S, 35 years; ■, 30 years; ♦, 25 years. From Janney et al. (62). Reproduced by permission of the American Society for Nutritional Sciences

Figure 20.8 Predicted weight retention curves for women of various ages (hypothetical married women who fully breast-fed for 6 months and gained 15.9 kg during pregnancy). •, 40 years; S, 35 years; ■, 30 years; ♦, 25 years. From Janney et al. (62). Reproduced by permission of the American Society for Nutritional Sciences

Figure 20.9 Independent relationships between weight gain and parity (data from Harris et al. (70). Adjusted for the effects of smoking status, alcohol consumption, socioeconomic status, breast-feeding at hospital, maternal age, nulliparous BMI, marital status, birth weight, plus (for analyses of inter-pregnancy weight gain only) gestational weight gain, and inter-pregnancy interval

Figure 20.9 Independent relationships between weight gain and parity (data from Harris et al. (70). Adjusted for the effects of smoking status, alcohol consumption, socioeconomic status, breast-feeding at hospital, maternal age, nulliparous BMI, marital status, birth weight, plus (for analyses of inter-pregnancy weight gain only) gestational weight gain, and inter-pregnancy interval experience pregnancy between the ages of 35 and 40 years, an average weight retention of up to 5 kg is predicted by 18 months postpartum. Ohlin and Rossner (54) also reported age to be more strongly related to long-term weight gain (P < 0.05) than was parity, and among primiparous women, those in the oldest age group ( > 36 years) retained significantly more weight (2.9 kg) than did younger women (1.4 kg). Given the current trend toward delayed childbearing (106), the risk of weight retention among mothers over the age of 35 certainly warrants further examination.

Parity

The interaction between parity and body weight is complex and highly confounded with maternal age because older women tend to have more children than younger women (25). After accounting for differences in maternal age, an effect of parity on body weight is only consistently observed at high parities (28,52,70).

Harris et al. (70) investigated the independent relationship between parity and maternal weight gain in a group of 523 multiparous women from south-east London. In this repeat-pregnancy study, the change in maternal body weight from the beginning of one pregnancy to the beginning of the next was examined, and parity was found to be indepen dently associated with both gestational weight gain and inter-pregnancy weight gain (Figure 20.9). These relationships suggest that first-time mothers are at risk of long-term weight gain because they gain the most weight during pregnancy, and high gestational weight gain is in itself a risk factor for long-term weight gain (49,50,57). However, women of higher parity (4 + ) seemed to be at risk of long-term weight gain because they gained more weight in association with pregnancy, irrespective of the amount of weight they gained during pregnancy. Therefore, for women of parity 3 or less, the association between maternal body weight and parity appeared to be the result of cumulative weight gains during successive pregnancies. For women of higher parity, the association between maternal body weight and parity was partly the cumulative effect of excess gestational weight gains from successive pregnancies, and partly the result of gaining more weight in association with later pregnancies.

It could be that the increase in weight gain observed with increasing parity is the result of women of higher parity having their pregnancies at older ages 'when weight gain is the norm' (66). However, more sophisticated analyses have shown the effects of parity on body weight to be independent of maternal age (28,70). Alternatively, mothers of high parity might gain more weight in association with pregnancy as a result of differential effects of motherhood at different parities. For example,

Dodge and Silva (107) showed that the pressures of child rearing increased with family size. In their study, the numbers of symptoms reported by mothers concerning physical ill health (including changes in body weight) were found to be positively correlated with the number of children living at home. Similarly, the numbers of symptoms concerning psychological ill health (including anxiety and depression) were significantly correlated with the number of children at home, especially in mothers with children of pre-school age. Since many women respond to stress by increasing their energy intake (85,86), this might explain why mothers with larger families tend to gain more weight than do mothers with fewer children. It is also conceivable that different demographic groups, who are at greater risk of weight gain, are selected into this high parity group and that the observed association is simply the result of uncontrolled confounding. The variable success of investigators to control for the effects of socioeconomic status might therefore explain why other researchers have found no effect of parity on weight development after pregnancy (49,54,60).

Ethnicity

To date, the relationship between ethnicity and maternal body weight has largely been restricted to a few national studies of white and black US women (48,57,69). These studies consistently show pregnancy-related weight gains to be greater in African-Americans than in white women (105), and at any level of weight gain, black women are seen to retain more weight postpartum than white women (48,57,58,61,69).

Data from the 1988 National Maternal and Infant Health Survey show that among women of normal BMI, black mothers are more than twice as likely than white mothers to retain at least 9 kg postpartum (69). This difference was shown to persist after adjustment for differences in maternal age, parity, gestational weight gain, infant birth weight, height, BMI, marital status and social class. The reasons for this differential impact of pregnancy on body weight among women from different ethnic backgrounds are not clear. A possible explanation has been suggested by researchers who examined national data on energy intakes before, during and after pregnancy (108). In this study, the reported energy intakes of non-lactating white women ap peared to decrease after pregnancy, whereas for black women, mean increases of more than 300 kcal over pre-pregnant intakes were reported by 3 months postpartum. This finding is consistent with others which suggest that black women are not necessarily at increased risk of gaining weight, but rather that they are less likely to lose it (58,109). Other studies have shown that factors related to postpartum weight retention differ by ethnic group: For example, married white mothers appear to have a lower risk of excess weight retention than unmarried white mothers, but among black mothers, marital status was unrelated to weight retention (69). In a similar way, high parity predicted retained weight for black but not white mothers (69). These and other observations suggest that ethnicity is probably just a proxy for social, economic, environmental, cultural and other factors that influence a woman's body weight following pregnancy.

Marital Status, Education, Income and Socioeconomic Status

Greater weight retention has been observed among unmarried women when compared to married women (49,62,68,69). Janney et al. (62) showed that unlike married women, unmarried women had a pattern of weight gain rather than weight loss, and Harris et al. (70), in their study of 523 multiparous English women, also showed marital status to be significantly related to long-term weight gain. After adjusting for the influence of confounding factors, Harris et al. (70) found that unmarried mothers retained significantly more weight (4.0 kg) than married mothers (3.15 kg) by at least 1 year postpar-tum. In this context it is likely that marital status serves as a proxy for socioeconomic status and/or social support. However, others have found no effect of marital status on long-term weight gain following pregnancy (54).

Marital status is just one of many component measures of socioeconomic status, and the relationships between the various measures of socioeconomic status and pregnancy-related weight gain are complex and inconsistent. Most studies show women with lower incomes or less education to be at increased risk of retaining weight postpar-tum (61,67,69,90). However, others that have measured socioeconomic status by occupation or social class have found no significant associations (49,50,54,70). As with many health outcomes, it is likely that the poorest members of society will be at greatest risk of maternal obesity.

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