Expert review
Obstetrics
The vascular anastomoses in monochorionic twin pregnancies and their clinical consequences

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Monochorionic twin pregnancies are at increased risk of adverse outcome because of the vascular anastomoses that connect the 2 fetal circulation systems. The shared circulation is responsible for some unique complications in monochorionic twins, such as the twin-to-twin transfusion syndrome, the twin anemia polycythemia sequence, the twin reversed arterial perfusion sequence, and monoamniotic twinning. Another consequence of the shared circulation is that the well-being of one twin critically depends on that of the other. In this review, we will describe the technique of placental injection. Further, we will discuss the role of the vascular anastomoses in each of the complications described above and provide an update on their management.

Section snippets

Injection studies of the monochorionic placenta

Placental injection from complicated monochorionic twin gestations often provides exceptionally useful information that would otherwise be lost on routine pathological examination. Nevertheless, placentas from pregnancies complicated with single death and delayed delivery can no longer be evaluated because of postmortem involution of the placental part of the demised twin.

Although placental injection is time-consuming, it is basically a simple technique that can be performed by anyone with

TTTS

TTTS is an antenatal sonographic diagnosis based on the presence of polyhydramnios (deepest vertical pocket ≥8 cm) with a distended bladder in the recipient and oligohydramnios (deepest vertical pocket ≤2 cm) with a small or empty bladder in the donor. Because amniotic fluid increases with gestation, most Europeans adhere to the gestational age-dependent cutoff of a deepest pocket ≥10 cm after 20 weeks to define the degree of polyhydramnios. “Twin-to-twin transfusion syndrome” is somewhat a

TAPS

In contrast to TTTS, which is essentially an amniotic fluid discordance, TAPS is characterized by a severe hemoglobin discordance caused by a chronic net transfusion over minuscule and usually unidirectional anastomoses. TAPS can be diagnosed antenatally based on discordant MCA-PSV measurements. One twin typically has an elevated MCA-PSV (>1.5 multiples of the median) and in the other twin the MCA-PSV is decreased (<1 multiples of the median).25 The anemic twin often has decreased amniotic

Conclusion

In summary, the vascular anastomoses are responsible for some unique complications in monochorionic twin pregnancies. They also account for the increased mortality and morbidity as compared to dichorionic twins, which have separate circulations. As long as the twins are connected to this shared circulation, intertwin transfusion imbalances may occur and each twin is exposed to the hormonal environment of its cotwin. The shared circulation is also responsible for the fact that the well-being of

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    J.D. is the recipient of a grant of Fonds voor Wetenschappelijk Onderzoek Vlaanderen (FWO; 1.8.012.07.N.02).

    The authors report no conflict of interest.

    Reprints not available from the authors.

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