The date of birth is most accurately indicated as 266 days, or 38 weeks, after fertilization. The oocyte is usually fertilized within 12 hours of ovulation. However, sperm deposited in the reproductive tract up to 6 days prior to ovulation can survive to fertilize oocytes. Thus, most pregnancies occur when sexual intercourse occurs within a 6-day period that ends on the day of ovulation. A
pregnant woman usually will see her obstetrician when she has missed two successive menstrual bleeds. By that time, her recollection about coitus is usually vague, and it is readily understandable that the day of fertilization is difficult to determine.
The obstetrician calculates the date of birth as 280 days or 40 weeks from the first day of the LNMP. In women with regular 28-day menstrual periods the method is fairly accurate, but when cycles are irregular, substantial miscalculations may be made. An additional complication occurs when the woman has some bleeding about 14 days after fertilization as a result of erosive activity by the implanting blastocyst (see Chapter 3). Hence the day of delivery is not always easy to determine. Most fetuses are born within 10 to 14 days of the calculated delivery date. If they are born much earlier, they are categorized as premature; if born later, they are considered postmature.
Occasionally the age of an embryo or small fetus must be determined. By combining data on the onset of the last menstrual period with fetal length, weight, and other morphological characteristics typical for a given month of development, a reasonable estimate of the age of the fetus can be formulated. A valuable tool for assisting in this determination is ultrasound, which can provide an accurate (1 to 2 days) measurement of CRL during the 7th to 14th weeks. Measurements commonly used in the 16th to 30th weeks are biparietal diameter (BPD), head and abdominal circumference, and femur length. An accurate determination of fetal size and age is important for managing pregnancy, especially if the mother has a small pelvis or the baby has a birth defect.
There is considerable variation in fetal length and weight, and sometimes these values do not correspond with the calculated age of the fetus in months or weeks. Most factors influencing length and weight are genetically determined, but environmental factors also play an important role.
Intrauterine growth restriction (IUGR) is a term applied to infants who are at or below the 10th percentile for their expected birth weight at a given gestational age. Sometimes these infants are described as small for dates, small for gestational age (SGA), fetally malnourished, or dysma-ture. Approximately 1 in 10 babies have IUGR and therefore an increased risk of neurological deficiencies, congenital malformations, meconium aspiration, hypoglycemia, hypocalcemia, and respiratory distress syndrome (RDS). The incidence is higher in blacks than in whites. Causative factors include chromosomal abnormalities (10%); teratogens; congenital infections (rubella, cytomegalovirus, toxoplasmosis, and syphilis); poor maternal health (hypertension and renal and cardiac disease); the mother's nutritional status and socioeconomic level; her use of cigarettes, alcohol, and other drugs; placental insufficiency; and multiple births (e.g., twins, triplets). Fetuses that weigh less
than 500 g seldom survive, while those that weigh 500 to 1000 g may live if provided with expert care. However, approximately 50% of babies born weighing less than 1000 g who survive will have severe neurological deficits. Infants may be full term, but small because of IUGR or small because they are born prematurely.
The major growth-promoting factor during development before and after birth is insulinlike growth factor-I (IGF-I), which has mitogenic and anabolic effects. Fetal tissues express IGF-I and serum levels are correlated with fetal growth. Mutations in the IGF-I gene result in IUGRand this growth retardation is continued after birth. In contrast to the prenatal period, postnatal growth depends upon growth hormone (GH). This hormone binds to its receptor (GHR), activating a signal transduction pathway and resulting in synthesis and secretion of IGF-I. Mutations in the GHR result in Laron dwarfism, which is characterized by growth retardation, midfacial hypoplasia, blue sclera, and limited elbow extension. These individuals show little or no IUGR, since IGF-I production does not depend upon GH during fetal development.
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