Monozygotic Twins

The second type of twins, which develops from a single fertilized ovum, is monozygotic, or identical, twins. The rate for monozygotic twins is 3 to 4 per 1000. They result from splitting of the zygote at various stages of development. The earliest separation is believed to occur at the two-cell stage, in which case two separate zygotes develop. The blastocysts implant separately, and each embryo has its own placenta and chorionic sac (Fig. 6.18A). Although the arrangement of the membranes of these twins resembles that of dizygotic twins, the two can be recognized as partners of a monozygotic pair by their strong resemblance in blood groups, fingerprints, sex, and external appearance, such as eye and hair color.

Splitting of the zygote usually occurs at the early blastocyst stage. The inner cell mass splits into two separate groups of cells within the same blastocyst cavity (Fig. 6.18B). The two embryos have a common placenta and a common chorionic cavity, but separate amniotic cavities (Fig. 6.18B). In rare cases the separation occurs at the bilaminar germ disc stage, just before the appearance of the primitive streak (Fig. 6.18C). This method of splitting results in formation of two partners with a single placenta and a common chorionic and amniotic sac. Although the twins have a common placenta, blood supply is usually well balanced.

Although triplets are rare (about 1/7600 pregnancies), birth of quadruplets, quintuplets, and so forth is rarer. In recent years multiple births have occurred more frequently in mothers given gonadotropins (fertility drugs) for ovulatory failure.

Placenta Electron
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Cleavage Embryonic Disc Conjoined Twins

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Figure 6.17 Development of dizygotic twins. Normally each embryo has its own amnion, chorion, and placenta, A, but sometimes the placentas are fused, B. Each embryo usually receives the appropriate amount of blood, but on occasion large anastomoses shunt more blood to one of the partners than to the other.

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Figure 6.17 Development of dizygotic twins. Normally each embryo has its own amnion, chorion, and placenta, A, but sometimes the placentas are fused, B. Each embryo usually receives the appropriate amount of blood, but on occasion large anastomoses shunt more blood to one of the partners than to the other.

Amniotic Cavity
Amniotic cavity
Conjoined Twins Cells SplitPlacenta Electron

Common placenta

Common chorionic cavity

Common amniotic cavity

Common placenta

Common amniotic cavity

Monozygotic Twins

Figure 6.18 Possible relations of fetal membranes in monozygotic twins. A. Splitting occurs at the two-cell stage, and each embryo has its own placenta, amniotic cavity, and chorionic cavity. B. Splitting of the inner cell mass into two completely separated groups. The two embryos have a common placenta and a common chorionic sac but separate amniotic cavities. C. Splitting of the inner cell mass at a late stage of development. The embryos have a common placenta, a common amniotic cavity, and a common chorionic cavity.

Figure 6.18 Possible relations of fetal membranes in monozygotic twins. A. Splitting occurs at the two-cell stage, and each embryo has its own placenta, amniotic cavity, and chorionic cavity. B. Splitting of the inner cell mass into two completely separated groups. The two embryos have a common placenta and a common chorionic sac but separate amniotic cavities. C. Splitting of the inner cell mass at a late stage of development. The embryos have a common placenta, a common amniotic cavity, and a common chorionic cavity.

CLINICAL CORRELATES Twin Defects

Twin pregnancies have a high incidence of perinatal mortality and morbidity and a tendency toward preterm delivery. Approximately 12% of premature infants are twins and twins are usually small at birth. Low birth weight and prematurity place infants of twin pregnancies at great risk, and approximately 10 to 20% of them die, compared with only 2% of infants from single pregnancies.

The incidence of twinning may be much higher, since twins are conceived more often than they are born. Many twins die before birth and some studies indicate that only 29% of women pregnant with twins actually give birth to two infants. The term vanishing twin refers to the death of one fetus. This disappearance, which occurs in the first trimester or early second trimester, may result from resorption or formation of a fetus papyraceus (Fig. 6.19).

Another problem leading to increased mortality among twins is the twin transfusion syndrome, which occurs in 5 to 15% of monochorionic

Figure 6.19 Fetus papyraceus. One twin is larger, and the other has been compressed and mummified, hence the term papyraceus.

monozygotic pregnancies. In this condition, placental vascular anastomoses, which occur in a balanced arrangement in most monochorionic placentas, are formed so that one twin receives most of the blood flow and flow to the other is compromised. As a result, one twin is larger than the other (Fig. 6.20). The outcome is poor, with the death of both twins occurring in 60 to 100% of cases.

At later stages of development, partial splitting of the primitive node and streak may result in formation of conjoined (Siamese) twins. These twins are classified according to the nature and degree of union as thoracopagus (pagos, fastened); pygopagus; and craniopagus (Figs. 6.21 and 6.22). Occasionally, monozygotic twins are connected only by a common skin bridge or by a common liver bridge. The type of twins formed depends upon when and to what extent abnormalities of the node and streak occurred. Mis-expression of genes, such as Goosecoid, may also result in conjoined twins. Many conjoined twins have survived, including the most famous pair, Chang and Eng, who were joined at the abdomen and who traveled to England and the United States on exhibitions in the mid-1800s. Finally settling in North Carolina, they farmed and fathered 21 children with their two wives.

Figure 6.20 Monozygotic twins with twin transfusion syndrome. Placental vascular anastomoses produced unbalanced blood flow to the two fetuses.

Conjoined Twin Zygote

Thoracopagus Pygopagus Craniopagus

Figure 6.21 Thoracopagus, pygopagus, and craniopagus twins. Conjoined twins can be separated only if they have no vital parts in common.

Thoracopagus Pygopagus Craniopagus

Figure 6.21 Thoracopagus, pygopagus, and craniopagus twins. Conjoined twins can be separated only if they have no vital parts in common.

Parturition (Birth)

For the first 34 to 38 weeks of gestation, the uterine myometrium does not respond to signals for parturition (birth). However, during the last 2 to 4 weeks of pregnancy, this tissue undergoes a transitional phase in preparation for the onset of labor. Ultimately, this phase ends with a thickening of the myometrium in the upper region of the uterus and a softening and thinning of the lower region and cervix.

Labor itself is divided into three stages: 1) effacement (thinning and shortening) and dilatation of the cervix; this stage ends when the cervix is fully dilated; 2) delivery of the fetus; and 3) delivery of the placenta and fetal membranes. Stage 1 is produced by uterine contractions that force the am-niotic sac against the cervical canal like a wedge or, if the membranes have ruptured, then pressure will be exerted by the presenting part of the fetus, usually the head. Stage 2 is also assisted by uterine contractions, but the most important force is provided by increased intra-abdominal pressure from contraction of abdominal muscles. Stage 3 requires uterine contractions and is aided by increasing intra-abdominal pressure.

As the uterus contracts, the upper part retracts creating a smaller and smaller lumen, while the lower part expands, thereby producing direction to the force. Contractions usually begin about 10 minutes apart; then, during the second stage of labor, they may occur less than 1 minute apart and last from 30 to 90 seconds. Their occurrence in pulses is essential to fetal survival, since they are of sufficient force to compromise uteroplacental blood flow to the fetus.

CLINICAL CORRELATES Preterm Birth

Factors initiating labor are not known and may involve: "retreat from maintenance of pregnancy" in which pregnancy supporting factors (e.g., hormones,

Figure 6.22 Conjoined twins. A. Twins with two heads, a broad thorax, two spines, two partially fused hearts, four lungs, and a duplicated gut down to the ileum. B. Twins joined at the head (craniopagus) with multiple deformations of the limbs.

etc.) are withdrawn; or active induction caused by stimulatory factors targeting the uterus. Probably, components of both phenomena are involved. Unfortunately, a lack of knowledge about these factors has restricted progress in preventing preterm birth. Preterm birth (delivery before 34 weeks) of premature infants is the second leading cause of infant mortality in the United States and also contributes significantly to morbidity. It is due to premature rupture of the membranes, premature onset of labor, or pregnancy complications requiring premature delivery. Maternal hypertension and diabetes as well as abruptio placenta are risk factors. Maternal infections, including bacterial vaginosis, are also associated with an increased risk.

Summary rThe fetal period extends from the ninth week of gestation until birth and is characterized by rapid growth of the body and maturation of organ systems. Growth in length is particularly striking during the third, fourth, and fifth months (approximately 5 cm per month), while increase in weight is most striking during the last 2 months of gestation (approximately 700 g per month) (Table 6.1; p. 118).

A striking change is the relative slowdown in the growth of the head. In the third month, it is about half of CRL. By the fifth month the size of the head is about one-third of CHL, and at birth it is one-fourth of CHL (Fig. 6.2).

During the fifth month, fetal movements are clearly recognized by the mother, and the fetus is covered with fine, small hair.

A fetus born during the sixth or the beginning of the seventh month has difficulty surviving, mainly because the respiratory and central nervous systems have not differentiated sufficiently.

In general, the length of pregnancy for a full-term fetus is considered to be 280 days, or 40 weeks after onset of the last menstruation or, more accurately, 266 days or 38 weeks after fertilization.

The placenta consists of two components: (a) a fetal portion, derived from the chorion frondosum or villous chorion, and (b) a maternal portion, derived from the decidua basalis. The space between the chorionic and decidual plates is filled with intervillous lakes of maternal blood. Villous trees (fetal tissue) grow into the maternal blood lakes and are bathed in them. The fetal circulation is at all times separated from the maternal circulation by (a) a syncytial membrane (a chorion derivative) and (b) endothelial cells from fetal capillaries. Hence the human placenta is of the hemochorial type.

Intervillous lakes of the fully grown placenta contain approximately 150 ml of maternal blood, which is renewed 3 or 4 times per minute. The villous area varies from 4 to 14 m2, facilitating exchange between mother and child.

Main functions of the placenta are (a) exchange of gases; (b) exchange of nutrients and electrolytes; (c) transmission of maternal antibodies, providing the fetus with passive immunity; (d) production of hormones, such as progesterone, estradiol, and estrogen (in addition, it produces hCG and somatomam-motropin); and (e) detoxification of some drugs.

The amnion is a large sac containing amniotic fluid in which the fetus is suspended by its umbilical cord. The fluid (a) absorbs jolts, (b) allows for fetal movements, and (c) prevents adherence of the embryo to surrounding tissues. The fetus swallows amniotic fluid, which is absorbed through its gut and cleared by the placenta. The fetus adds urine to the amniotic fluid, but this is mostly water. An excessive amount of amniotic fluid (hydramnios) is associated with anencephaly and esophageal atresia, whereas an insufficient amount (oligohydramnios) is related to renal agenesis.

The umbilical cord, surrounded by the amnion, contains (a) two umbilical arteries, (b) one umbilical vein, and (c) Wharton's jelly, which serves as a protective cushion for the vessels.

Fetal membranes in twins vary according to their origin and time of formation. Two-thirds of twins are dizygotic, or fraternal; they have two amnions, two chorions, and two placentas, which sometimes are fused. Monozygotic twins usually have two amnions, one chorion, and one placenta. In cases of conjoined twins, in which the fetuses are not entirely split from each other, there is one amnion, one chorion, and one placenta.

Signals initiating parturition (birth) are not clear, but preparation for labor usually begins between 34 and 38 weeks. Labor itself consists of three stages: 1) effacement and dilatation of the cervix; 2) delivery of the fetus; and 3) delivery of the placenta and fetal membranes.

Problems to Solve

1. An ultrasound at 7 months of gestation shows too much space (fluid accumulation) in the amniotic cavity. What is this condition called, and what are its causes?

2. Later in her pregnancy a woman realizes that she was probably exposed to toluene in the workplace during the third week of gestation but tells a fellow worker that she is not concerned about her baby because the placenta protects her infant from toxic factors by acting as a barrier. Is she correct?

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Bacterial Vaginosis Facts

Bacterial Vaginosis Facts

This fact sheet is designed to provide you with information on Bacterial Vaginosis. Bacterial vaginosis is an abnormal vaginal condition that is characterized by vaginal discharge and results from an overgrowth of atypical bacteria in the vagina.

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Responses

  • Lyle Pullman
    Can a perforated sac of vanished twin hurt other fetus?
    8 years ago
  • Fosco
    How conjoined twins occur?
    7 years ago
  • jasmine
    How twins are suspended in the amniotic cavity of mother?
    4 years ago
  • jill
    When a 7 months gestation shows too much space or fluid accumulation what is the condition called?
    3 years ago

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