Multiple Gestation

Multiple gestation is a pregnancy carrying more than one fetus, most commonly twins. It is classified by the number of fetuses and, critically, by chorionicity and amnionicity, which describe how the placentas and amniotic sacs are shared. Multiple gestation raises the risk of nearly every major pregnancy complication and is therefore a core concern of high-risk maternal care.

Definition

Multiple gestation is a pregnancy with two or more fetuses; it is characterized by zygosity (whether fetuses arise from one or more than one fertilized egg) and, for clinical risk, by chorionicity and amnionicity, which describe whether the fetuses share a placenta and an amniotic sac.

Scope

This entry covers the classification of multiple pregnancy by zygosity, chorionicity, and amnionicity; why chorionicity is the key determinant of risk; the complications unique to monochorionic pregnancies such as twin-to-twin transfusion syndrome; and the elevated background risk of preterm birth and other shared complications. It is a reference and educational overview and does not provide surveillance schedules, timing-of-delivery recommendations, or any individualized management guidance.

Core questions

• How are multiple pregnancies classified by zygosity, chorionicity, and amnionicity?
• Why is chorionicity the most important determinant of risk in twin pregnancy?
• What complications are specific to monochorionic pregnancies?
• Why does multiple gestation increase the risk of preterm birth and other complications?

Key concepts

• Zygosity (monozygotic vs dizygotic)
• Chorionicity (monochorionic vs dichorionic)
• Amnionicity
• Twin-to-twin transfusion syndrome (TTTS)
• Selective fetal growth restriction
• Shared placental circulation (vascular anastomoses)
• Uterine overdistension
• Higher-order multiples

Mechanisms

The risks of multiple gestation flow largely from how the placentas are shared. Dizygotic twins always have separate (dichorionic) placentas, whereas monozygotic twins may be dichorionic or monochorionic depending on the timing of the splitting of the embryo. In monochorionic pregnancies the twins share a single placenta with vascular anastomoses, which can produce imbalanced flow and complications such as twin-to-twin transfusion syndrome and selective fetal growth restriction (Khalil, 2016). Independently of chorionicity, carrying more than one fetus distends the uterus and increases physiologic demand, which contributes to the markedly higher rate of preterm birth and to elevated risks of hypertensive disorders and hemorrhage (Cunningham et al., 2022).

Clinical relevance

Multiple gestation is one of the clearest reasons a pregnancy is managed as high-risk, and accurate early determination of chorionicity by ultrasound underpins the intensity of subsequent surveillance. This entry explains how multiple pregnancies are classified and why chorionicity matters; it is not a guide to monitoring schedules, intervention thresholds, or timing of delivery, which rest with the responsible clinical team following current guidelines.

Epidemiology

Twin and higher-order multiple births rose substantially in many countries with the expansion of assisted reproductive technology and older maternal age, though single-embryo transfer policies have moderated this trend in some settings. Monochorionic pregnancies, although a minority of twins, account for a disproportionate share of complications because of their shared circulation (Khalil, 2016).

Evidence & guidelines

The ISUOG practice guidelines define the role of ultrasound in determining chorionicity and surveilling twin pregnancy (Khalil, 2016). The Eurofetus randomized trial established laser coagulation of placental anastomoses as superior to serial amnioreduction for severe twin-to-twin transfusion syndrome (Senat, 2004), and twin-specific growth references have been derived from cohort data (Stirrup, 2015). Standard obstetric texts summarize classification and risk (Cunningham et al., 2022).

History

Twins were long classified mainly by zygosity, but the recognition that chorionicity, rather than zygosity, governs the most serious risks reoriented clinical practice toward early ultrasound determination of placentation. The development of fetoscopic laser therapy for twin-to-twin transfusion syndrome, validated by the Eurofetus trial (Senat, 2004), marked a major advance in the management of monochorionic complications.

Seminal works

• khalil-2016
• senat-2004

Frequently asked questions

Why does chorionicity matter more than whether twins are identical?

Chorionicity describes whether twins share a single placenta; monochorionic twins share placental blood vessels and so face risks such as twin-to-twin transfusion syndrome that do not arise when each twin has its own placenta, regardless of whether the twins are identical.

Why is multiple gestation considered high-risk?

Carrying more than one fetus markedly increases the risk of preterm birth and raises the risk of hypertensive disorders, hemorrhage, and, in monochorionic pregnancies, complications of shared circulation, which is why these pregnancies receive intensified surveillance.