Multifetal Gestation and Malpresentation

Chapter 13 Multifetal Gestation and Malpresentation




image Multiple Gestation


Multiple gestation is defined as any pregnancy in which two or more embryos or fetuses occupy the uterus simultaneously. It is of utmost importance to recognize multiple gestation as a complication of pregnancy. Because the mean gestational age of delivery of twins is about 36 weeks, the perinatal mortality and morbidity in multiple gestation exceeds that of singletons disproportionately. Because of the additional physiologic stresses associated with two fetuses and placentas and a rapidly enlarging uterus, maternal morbidity is also increased.



ETIOLOGY AND CLASSIFICATION OF TWINNING


Multiple gestation occurs as the result of either 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 dichorionic, diamniotic 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-layer 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 for umbilical cord entanglement is high, resulting in a net mortality in these twins of almost 50% (Figure 13-1).


TABLE 13-1 RELATIONSHIP BETWEEN TIMING OF CLEAVAGE AND NATURE OF MEMBRANES IN TWIN GESTATIONS















Time of Cleavage Nature of Membranes
0-72 hr Dichorionic, diamniotic
4-8 days Monochorionic, diamniotic
9-12 days Monochorionic, monoamniotic

Time interval between ovulation and cleavage of the egg.





DETERMINATION OF ZYGOSITY


The prognosis and expected morbidity with twins is strongly dependent on zygosity: monozygotic twins are more likely to involve congenital anomalies, weight discordancy, twin-twin transfusion syndrome (TTTS), 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-chorion septum is suggestive of dizygotic twins, as is the presence of a “peak” or inverted “V” at the base of the membrane septum (Figure 13-2A). Conversely, in monochorionic gestation, the dividing membrane is fairly thin (Figure 13-2B). 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, confident 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 have the same sex, dichorionic placentas, but different blood groupings, and must be, therefore, dizygotic. Twenty percent 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, 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, 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 one twin’s circulation perfuses the other (i.e., TTTS) in about 10% of monozygotic twins. In this syndrome, arterial blood from the “donor twin” enters the placenta (through 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 for demise from the circulatory derangement, and the pregnancy is predisposed further for preterm delivery due to uterine overdistention with hydramnios.


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



Given the poor prognosis of untreated TTTS (about 50% survival of either twin), treatment with either serial amniocentesis and fluid reduction from the recipient twin’s sac or laser photocoagulation of the anastomotic vessels on the surface of the placenta is performed in specialized centers.







Jun 4, 2016 | Posted by in GYNECOLOGY | Comments Off on Multifetal Gestation and Malpresentation

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