Multiple pregnancy and twin–twin transfusion syndrome !

Pregnancy with two or more babies. It carries more risks for mother and babies, and identical twins sharing one placenta can develop twin-to-twin transfusion.

Updated 2026-11-01

Structures involved

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Why it happens

What it is

A pregnancy with more than one fetus. Twins may be dizygotic (two eggs, always dichorionic) or monozygotic (one egg that splits). Chorionicity, meaning how many placentas the fetuses share, matters more than zygosity: monochorionic twins share one placenta with vascular connections and face specific complications such as twin–twin transfusion syndrome (TTTS).

Who gets it

Twins make up a few percent of births. Monozygotic twinning is fairly constant worldwide, while dizygotic twinning varies with ethnicity, maternal age and assisted reproduction, whose growth has raised multiple-birth rates in Colombian and Latin American cities. TTTS affects about one in ten monochorionic diamniotic twin pregnancies.

Causes

Release and fertilisation of two or more eggs (dizygotic twins)
Division of a single embryo (monozygotic twins); timing of division determines chorionicity and amnionicity
Ovulation induction and assisted reproductive techniques
TTTS: unbalanced arteriovenous connections across a shared placenta

Risk factors

Fertility treatment (ovulation induction, in vitro fertilisation)
Maternal age over 30 to 35
Family history of dizygotic twins on the mother's side
Higher parity

Pathophysiology

The uterus is overdistended and the placental mass is larger, which increases the risk of preterm labour, preeclampsia, gestational diabetes and postpartum uterine atony. In monochorionic twins, placental anastomoses link the two circulations. When flow through deep artery-to-vein connections is unbalanced, the donor twin becomes hypovolaemic, oliguric and growth restricted with oligohydramnios ("stuck twin"), while the recipient becomes volume overloaded, polyuric, with polyhydramnios, cardiomegaly and possible hydrops.

Classification

Dichorionic diamniotic (two placentas, two sacs; lowest risk)
Monochorionic diamniotic (one placenta, two sacs; risk of TTTS)
Monochorionic monoamniotic (one sac; risk of cord entanglement)
Conjoined twins (very late division)
TTTS Quintero stages I to V

Symptoms and signs

Uterus larger than expected for dates
More intense nausea and vomiting of pregnancy
More fetal movements felt in several places
In TTTS: sudden increase in abdominal size, tightness or breathlessness from polyhydramnios

Diagnosis

Clinical: Suspected by a uterus large for dates or two fetal heart beats; confirmed by ultrasound.
Tests: Usual antenatal tests with closer screening for anaemia, gestational diabetes and preeclampsia. hCG and alpha-fetoprotein are higher than in singletons, so screening results must be interpreted for twins.
Imaging: First-trimester ultrasound to determine chorionicity (lambda or twin-peak sign in dichorionic twins, T sign in monochorionic twins). Monochorionic twins need ultrasound every two weeks from about 16 weeks to look for amniotic fluid discordance (TTTS), growth discordance and Doppler changes.

Differential diagnosis

Wrong gestational dates
Polyhydramnios of other causes
Uterine fibroids
Fetal macrosomia
Molar pregnancy

Goal of treatment

Know the chorionicity early, prevent and detect complications, treat TTTS in time, and plan the timing and route of delivery to reduce preterm birth and perinatal death.

Treatment

Follow-up: More frequent antenatal visits and serial ultrasound for growth, fluid and Doppler; screening for anaemia, glucose intolerance and hypertension.
Medical: Antenatal corticosteroids for fetal lung maturity if preterm birth is expected, and low-dose aspirin for preeclampsia prevention when risk criteria are met; iron and folic acid supplementation.
Procedure: TTTS: fetoscopic laser coagulation of the placental anastomoses is the preferred treatment for advanced stages before about 26 weeks; serial amnioreduction is an alternative.
Surgical: Delivery planned by chorionicity: vaginal birth is possible when the first twin is cephalic; caesarean section when the first twin is non-cephalic, in monoamniotic twins and in most higher-order multiples.
Follow-up: Education on signs of preterm labour, nutrition and rest, and preparation for postpartum haemorrhage (active management of the third stage).

Complications

Preterm birth and low birth weight
Preeclampsia and gestational diabetes
Fetal growth restriction and growth discordance
TTTS, twin anaemia-polycythaemia sequence and death of one twin
Cord entanglement in monoamniotic twins
Postpartum haemorrhage from uterine atony

Prognosis

Most dichorionic twins do well, though preterm birth is common. Monochorionic pregnancies have higher perinatal loss; untreated severe TTTS has a very high mortality, while laser treatment allows at least one twin to survive in most cases. Death of one monochorionic twin can injure the brain of the survivor.

Prevention

Limiting the number of embryos transferred in assisted reproduction
Early ultrasound to define chorionicity
Scheduled ultrasound surveillance for monochorionic twins

Test yourself

Think of the answer, then tap the question to see it.

Which twin type is at risk of twin–twin transfusion syndrome?

Monochorionic twins (one shared placenta), mainly monochorionic diamniotic.

In TTTS, which twin has polyhydramnios?

The recipient twin; the donor has oligohydramnios and growth restriction.

Which ultrasound sign indicates dichorionic twins?

The lambda (twin-peak) sign; the T sign suggests monochorionic.

What is the preferred treatment for advanced TTTS before about 26 weeks?

Fetoscopic laser coagulation of the placental anastomoses.

What is the most common complication of multiple pregnancy?

Preterm birth.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: Twins, Triplets, Multiple Births (MedlinePlus (U.S. National Library of Medicine), 2024); Twin-to-twin transfusion syndrome (MedlinePlus (U.S. National Library of Medicine), 2024).

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Representative model, for educational purposes only.

Body of uterus highlighted on the Durgical 3D model