Zygosity

TTTS. Zygosity

Pregnancies complicated by TTTS are, with rare exception, monozygotic twins. Monozygotic twinning results from an early (within 14 days following fertilization) embryonic ‘splitting’ of a single zygote. The cause of this division is unknown, although some have viewed it as an anomalous embryonic event.

The timing of the division determines both the chorionicity and amnionicity.7 Division prior to the differentiation of the chorion results in the development of a dichorionic–diamniotic twin gestation; this occurs in approximately 30% of monozygotic twins. 

Twinning after differentiation of the chorion but before the amnion results in a monochorionic– diamniotic twin gestation. This constitutes 60–70% of monozygotic twins, and is the placental type commonly seen in TTTS. A monochorionic– monoamniotic twin gestation ensues after the differentiation of the amnion in about 1% of monozygotic twins. TTTS typically occurs in 5–17% of monochorionic-diamniotic placentas, and although extremely rare, the syndrome has also been noted in dichorionic-diamniotic and monochorionic-monoamniotic placentas. 

Although monozygotic twins typically have identical genetic make-up, phenotypic discordance is not uncommon.8 Phenotypic discordance is presumed to occur if variable influences are exerted on the developing twins. Examples of placental factors that may facilitate discordance in monochorionic twins include abnormal vascular communications, unequal partitioning of the placental mass, or variable locations of the placental cord insertions. 

These factors are addressed in subsequent sections of this chapter. Although rare, a handful of reported cases describe dizygotic monochorionic twin gestations. 9,10 This is hypothesized to occur from fusion of separately fertilized embryos such that the outer cells, which have committed to form the trophoblast, intermix to form a single placenta. Quintero et al reported a case of dizygotic monochorionic–diamniotic twins complicated by TTTS. The twins were discordant for gender. Microsatellite analysis of pericentromeric markers suggested dispermic fertilization of two separate ova. These findings suggest that chorionicity, not zygosity, may be responsible for placental vascular anastomoses.

However, our group has also noted one case of ‘dichorionic’ twins with TTTS. Fetoscopic surgery identified a single vascular communication. Pathological study of the fetal dividing membranes identified one area in which the chorion became progressively diminutive and then disappeared. We hypothesized that the chorionicity was a hybrid, with a region consistent with monochorionicity in a predominantly dichorionic placentation. Other groups have also reported cases of monochorionic twins with two completely separate placental disks connected by a thin bridge of placental tissue.11 In an unpublished study of 600 MC placentas, 

Machin reports having seen eight cases of bipartite monochorionic placentas.12 Recently, Lopriore et al described three bipartite placentas in a consecutive series of 109 monochorionic placentas (Figure 1).13 Both studies imply that the incidence of bipartite monochorionic placentas (1 to 3%) is not as rare as previously suspected.