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Multiple pregnancy (twins)
In a twin pregnancy the first thing to establish is chorionicity, in the first trimester: it sets the risks (twin-to-twin transfusion occurs only with a shared placenta), the frequency of surveillance and the timing of delivery.
Why it matters in the exam
Twin questions come back to three decisions. The first is chorionicity on ultrasound before 14 weeks: the lambda sign indicates two placentas, and the T sign a shared placenta. The second is surveillance: monochorionic twins are scanned fortnightly starting at 16 weeks, to pick up twin-to-twin transfusion syndrome (TTTS) early — a donor twin with oligohydramnios and a recipient twin with polyhydramnios. The third is the timing and mode of delivery, which depend on the type of pregnancy and the presentation of the first twin.
Remember the maternal complications too: preterm birth (the most common complication), preeclampsia — so low-dose aspirin is recommended for every twin pregnancy — gestational diabetes, anaemia and postpartum haemorrhage. Another classic question concerns the timing of division of the fertilised egg in identical twins: the later the split, the more structures the fetuses share, and the higher the risk. Monochorionic monoamniotic twins are at risk of cord entanglement and are delivered early by caesarean. When the first twin is not cephalic, delivery is usually by caesarean section.
Key points
- Chorionicity is set before 14 weeks: lambda = two placentas, T = shared placenta.
- TTTS only with a shared placenta; surveillance every two weeks from 16 weeks; laser treatment.
- Delivery: DCDA at 38 weeks; MCDA at 34 to 37+6 weeks; MCMA at 32 to 34 weeks by caesarean.
- First twin cephalic → vaginal delivery is possible.
- Low-dose aspirin for every twin pregnancy.
Summary table
| Type | Time of split (identical twins) | Main risk | Recommended delivery timing |
|---|---|---|---|
| Dichorionic diamniotic (DCDA) | Up to day 3 (or non-identical twins) | Preterm birth, preeclampsia | 38 to 38+6 weeks |
| Monochorionic diamniotic (MCDA) | Days 4–8 | TTTS, growth discordance, TAPS | 34 to 37+6 weeks |
| Monochorionic monoamniotic (MCMA) | Days 8–12 | Cord entanglement | 32 to 34 weeks, by caesarean |
| Conjoined twins | After day 13 | Complex anomalies | Planned caesarean in a specialist centre |
| TTTS | Only with a shared placenta | Donor: oligohydramnios; recipient: polyhydramnios and heart failure | Laser ablation of placental vessels at 16 to 26 weeks |
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Sources
- ACOG Practice Bulletin No. 231, 2021 — Multifetal Gestations: Twin, Triplet, and Higher-Order Multifetal Pregnancies
- ACOG Committee Opinion No. 831, 2021 — Medically Indicated Late-Preterm and Early-Term Deliveries
- ISUOG Practice Guidelines, 2016, updated 2025 — Role of ultrasound in twin pregnancy
- Williams Obstetrics, 26th edition (2022) — Multifetal Pregnancy
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.