Multiple Gestations and Complications

A multiple gestation is a pregnancy carrying more than one fetus, most commonly twins. These pregnancies carry distinctive risks that depend heavily on chorionicity, the number of placentas, with monochorionic twins, who share a single placenta, exposed to complications that do not occur in singleton or dichorionic pregnancies.

Definition

Multiple gestation denotes the simultaneous presence of two or more fetuses in a pregnancy, classified by zygosity (the number of fertilized eggs) and, more consequentially for risk, by chorionicity and amnionicity (the number of placentas and amniotic sacs), which determine whether the fetuses share placental circulation.

Scope

The entry covers the determination and importance of chorionicity, the principal complications of multiple gestations, the placental vascular basis of twin-twin transfusion syndrome, and the heightened risk of preterm birth and growth disturbance. It is a reference topic and does not provide guidance on surveillance intervals or delivery management.

Core questions

• Why is chorionicity, rather than zygosity, the key determinant of risk?
• What complications are specific to monochorionic twins?
• How do placental vascular anastomoses give rise to twin-twin transfusion syndrome?
• Why are multiple gestations at greater risk of preterm birth and growth restriction?

Key concepts

• Zygosity versus chorionicity
• Monochorionic and dichorionic placentation
• Amnionicity
• Twin-twin transfusion syndrome
• Selective fetal growth restriction
• Placental vascular anastomoses
• Preterm birth risk

Mechanisms

In monochorionic twins, the shared placenta contains vascular anastomoses that connect the two fetal circulations. Imbalanced flow across these connections underlies twin-twin transfusion syndrome, in which one twin (the donor) becomes hypovolemic and oligohydramniotic while the other (the recipient) becomes hypervolemic and polyhydramniotic. Unequal sharing of placental territory contributes to selective fetal growth restriction. These shared-circulation phenomena cannot occur in dichorionic pregnancies, where each fetus has its own placenta, which is why accurate determination of chorionicity, best done by early ultrasound, is fundamental (Lewi, 2022; Khalil et al., 2016).

Clinical relevance

Identifying multiple gestation and its chorionicity structures antenatal surveillance and underlies recognition of twin-specific complications, and understanding these entities supports appraisal of obstetric evidence. This entry describes the biology and complications; it is not a basis for individual decisions about monitoring or timing of delivery.

Epidemiology

Multiple gestations account for a small fraction of pregnancies but a disproportionate share of perinatal morbidity and mortality, driven largely by preterm birth, to which they are strongly predisposed (Goldenberg et al., 2008). Their frequency has risen in many countries with assisted reproduction and advancing maternal age.

Evidence & guidelines

ISUOG provides guidance on the ultrasound assessment of twin pregnancies, including determination of chorionicity and surveillance for twin-specific complications (Khalil et al., 2016). A landmark randomized trial showed that fetoscopic laser coagulation of placental anastomoses improved outcomes over serial amnioreduction in severe twin-twin transfusion syndrome, establishing laser therapy as the reference treatment (Senat et al., 2004).

History

Recognition that monochorionic twins share placental vasculature reframed twin pregnancy around chorionicity rather than zygosity. The development of early sonographic chorionicity assessment and, in the early 2000s, of fetoscopic laser surgery for twin-twin transfusion syndrome marked major advances in the antenatal care of multiple gestations.

Seminal works

• senat-2004
• khalil-2016

Frequently asked questions

Why does chorionicity matter more than whether twins are identical?

Chorionicity determines whether the twins share a placenta and therefore a connected circulation; monochorionic twins, who share one placenta, face complications such as twin-twin transfusion syndrome that cannot occur when each fetus has its own placenta, making chorionicity the key driver of risk.

What is twin-twin transfusion syndrome?

It is a complication of monochorionic twin pregnancies in which unbalanced blood flow through shared placental vessels leaves one twin underperfused with low fluid and the other overperfused with excess fluid; severe cases are treated by laser coagulation of the connecting vessels.