Multifetal Gestation and Malpresentation

Etiology and Classification of Twinning

Multiple gestations occur either as the result of the splitting of an embryo (i.e., identical or monozygotic twinning ) or the fertilization of two or more eggs produced in a single menstrual cycle (i.e., fraternal or dizygotic twinning ). Because dizygotic twins arise from separate eggs, they are structurally distinct pregnancies coexisting in a single uterus, each with its own amnion, chorion, and placenta. Monozygotic twins arise from cleavage of a single fertilized egg at various stages during embryogenesis, and thus the arrangement of the fetal membranes and placentas will depend on the time at which the embryo divides ( Table 13-1 ). The earlier the embryo splits, the more separate the membranes and placentas will be. If division occurs within the first 72 hours of fertilization, the membranes will be diamniotic, dichorionic with a thick, four-layered intervening membrane. If division occurs after 4 to 8 days of development, when the chorion has already formed, monochorionic, diamniotic twins will evolve with a thin, two-layered septum. If splitting occurs after 8 days, when both amnion and chorion have already formed, the result will be monochorionic, monoamniotic, twins residing in a single sac with no septum. Of all monozygotic twins, 30% are dichorionic, diamniotic, and 69% are monochorionic, diamniotic. Only 1% of twins are monoamnionic. Because twins share a sac in this type, without an intervening membrane, the risk of umbilical cord entanglement is high, resulting in a net mortality in these twins of almost 50% ( Figure 13-1 ).

Incidence and Epidemiology

Twins account for approximately 3.5% of all births in the United States. The frequency of monozygotic twinning, which depends on a very infrequent biologic event (embryo splitting), is constant in all populations studied at about 1 in 250 births. However, the frequency of dizygotic twinning, which arises from multiple ovulations in the mother, is strongly influenced by family history, ethnicity, and maternal age. A family history of dizygotic but not monozygotic twins in the maternal pedigree increases the likelihood of dizygotic twinning in subsequent generations. In western Nigeria, twinning occurs in 1 of 22 gestations, whereas in the Native American and Inuit populations, twinning is less than one-fifth of that rate. Twins are twice as common in women over 35 as in women at 25 years of age. Given these statistics, approximately two-thirds of spontaneously conceived twins are fraternal and one-third are identical (monozygotic). However, in recent years, the incidence of multizygotic multifetal gestation has increased markedly with the more widespread use of ovulation induction agents and the practice of transferring multiple embryos after in vitro fertilization. The incidence of multiple gestations following the use of clomiphene is about 6-8% and about 20-30% following gonadotropin therapy.

Determination of Zygosity

The prognosis and expected morbidity of twins is strongly dependent on zygosity: monozygotic twins are more likely to have congenital anomalies, weight discordance, twin-twin transfusion syndrome, neurologic morbidity, premature delivery, and fetal death. Thus, determination of zygosity is the most important next step after multifetal pregnancy has been first diagnosed.

Ultrasonographic evaluation of the pregnancy is frequently very helpful in determining zygosity. Imaging of discordant fetal gender confirms a dizygotic gestation. Visualization of a thick amnion-chorionic septum is suggestive of dizygotic twins, as is the presence of a “peak” or inverted “V” at the base of the membrane septum ( Figure 13-2, A ). Conversely, in a monochorionic gestation, the dividing membrane is fairly thin (see Figure 13-2, B ). Because an early embryonic split can infrequently result in dichorionic, diamniotic twins with separate placentas, these findings are not definitive. Similarly, in rare cases of postzygotic genetic events, monochorionic twins may be gender discordant. Thus, definitive diagnosis of zygosity may require detailed examination of the placenta after delivery. Thirty percent of twins will be of different sex and are, therefore, dizygotic. Twenty-three percent have monochorionic placentas and are, therefore, monozygotic. Twenty-seven percent will have the same sex, dichorionic placentas, but different blood groupings, and must be, therefore, dizygotic. Twenty percent will have the same sex, dichorionic placentas, and identical blood groupings. For the latter group, further studies, such as human leukocyte antigen (HLA) typing or DNA analysis, will be required to allow determination of zygosity.

Abnormalities of the Twinning Process

Among monozygotic multiple gestations, abnormalities in the twinning process are relatively common and include conjoined twins, interplacental vascular anastomoses, twin-twin transfusion syndrome (TTTS), fetal malformations, and umbilical cord abnormalities.

Twin-Twin Transfusion Syndrome

The presence of unbalanced anastomoses in the placenta (typically arterial-venous connections) leads to a syndrome in which the circulation of one twin perfuses that of the other (i.e., TTTS) in approximately 10% of monozygotic twins. In this syndrome, arterial blood from the “donor twin” enters the placenta (via the umbilical artery) and is taken up by the umbilical venous system belonging to the “recipient twin,” which results in a net transfer of blood from the “donor” to the recipient twin. Fetal complications include hypovolemia, hypotension, anemia, oligohydramnios, and growth restriction in the donor twin, and hypervolemia, hydramnios, hyperviscosity, thrombosis, hypertension, cardiomegaly, polycythemia, edema, and congestive heart failure in the recipient twin. Both twins are at risk of demise from the circulatory derangement, and the pregnancy is predisposed to preterm delivery due to overdistention of the uterus with hydramnios.

Twin to twin transfusion is diagnosed using ultrasound. Typically, the donor twin is smaller and may have oligohydramnios, absent bladder, and anemia. The recipient twin, on the other hand, is larger with possible polyhydramnios, cardiomegaly, and ascites or hydrops ( Figure 13-3 ).

Given the poor prognosis of untreated TTTS (approximately 50% survival of any twin), treatment with either serial amniocenteses with fluid reduction from the recipient twin's sac, or laser photocoagulation of the anastomotic vessels on the surface of the placenta, are performed in specialized centers.

Altered Maternal Physiologic Adaptation with Multiple Fetuses

A number of normal maternal physiologic responses to pregnancy are exaggerated with multiple gestations. Whereas in normal pregnancy, maternal blood volume is augmented by 40% (2 L over the nonpregnant baseline), in twins this increase may be 3 L or more. The increased blood volume and demand for iron and folate increase the risk of anemia in the mother and make the patient less able to tolerate the stresses of infection, labor, and premature labor. Preeclampsia and gestational hypertension are almost doubled in multifetal gestation. The increased uterine size associated with multiple fetuses can cause maternal respiratory embarrassment, orthostatic hypotension due to compression of the vena cava, and compromised renal function due to compression of the ureters.

Diagnosis

Historical factors such as a maternal family history of dizygotic twinning, the use of fertility drugs, a maternal sensation of feeling larger than with previous pregnancies, or a sensation of excessive fetal movements should raise the suspicion of twins. Physical signs, including excessive weight gain, excessive uterine fundal growth, and auscultation of fetal hearts in separate quadrants of the uterus are suggestive but not diagnostic. An obstetric ultrasound should be performed when a multiple gestation is suspected. The diagnosis of multiple gestations requires a sonographic examination demonstrating two separate fetuses and heart activities, and can be made as early as 6 weeks' gestation.