Multiple Gestation
Chorionicity is destiny: it sets the risks, the scan schedule, and the delivery date.
Zygosity and chorionicity
Dizygotic twins. Two ova are released and fertilized by two sperm, so each twin develops its own chorion and amnion: dizygotic twins are always dichorionic-diamniotic (DC/DA), although the two placentas may fuse. About two-thirds of twins are dizygotic. Their rate rises with assisted reproduction (ovulation induction, IVF), advanced maternal age, a maternal family history of twins, and higher parity.
Monozygotic twins. A single embryo divides. The monozygotic rate is roughly constant worldwide (about 1 in 250 births) and is not inherited. The day of division determines which membranes are shared, and the later the split, the more the twins share and the higher the risk:
| Division | Result | Share of monozygotic twins |
|---|---|---|
| Days 0–3 (before the chorion forms) | DC/DA | ~25–30% |
| Days 4–8 (after chorion, before amnion) | MC/DA | ~70% |
| Days 8–12 | MC/MA | ~1% |
| Day 13 or later | Conjoined twins | Rare |
Ultrasound. Chorionicity is best determined in the first trimester. In the lambda (twin-peak) sign, a wedge of chorionic tissue projects into the base of a thick dividing membrane, meaning dichorionic twins. In the T sign, a thin membrane meets a single placenta at a right angle, meaning monochorionic twins. Different fetal sexes or two separate placentas also prove dichorionicity. No dividing membrane at all means monoamniotic twins.
Maternal and fetal risks
Mother. Preterm birth is the main driver of twin morbidity, and about 60% of twins deliver before 37 weeks. Twin pregnancy also raises the risk of preeclampsia, gestational diabetes, anemia (larger plasma volume expansion and higher iron and folate needs), hyperemesis (higher hCG), postpartum hemorrhage from an overdistended, atonic uterus, and cesarean delivery. Multifetal gestation alone is an indication for low-dose aspirin, started at 12–28 weeks and ideally before 16. Prophylactic cerclage, progesterone, and bed rest do not prevent preterm birth in unselected twin pregnancies.
Fetuses. Congenital anomalies are more common in monozygotic twins. Growth restriction and growth discordance of 20% or more (the EFW difference divided by the larger twin's EFW) call for closer surveillance.
A single fetal death is far more dangerous to the survivor in monochorionic twins: blood shifts acutely from the living twin into the dead twin's low-pressure circulation across shared anastomoses, causing death or brain injury. The injury occurs at the moment of demise, so immediate delivery does not prevent it.
Complications of a shared placenta
Surveillance. Monochorionic twins get ultrasound every 2 weeks from 16 weeks, checking fluid pockets, bladders, growth, and middle cerebral artery Doppler. Uncomplicated dichorionic twins need growth scans every 4–6 weeks from about 20 weeks.
Twin-twin transfusion syndrome. In about 10–15% of MC/DA pregnancies, unbalanced arteriovenous anastomoses shunt blood from a donor to a recipient. The donor becomes hypovolemic and oliguric, with oligohydramnios (a "stuck twin" pressed against the uterine wall with an empty bladder), anemia, and growth restriction. The recipient becomes hypervolemic and polyuric, with polyhydramnios, polycythemia, cardiomegaly, heart failure, and eventually hydrops. Diagnosis requires fluid discordance: the donor's deepest pocket under 2 cm and the recipient's at 8 cm or more. Quintero staging adds a nonvisible donor bladder, abnormal Doppler, hydrops, and demise.
| Complication | Mechanism | Management |
|---|---|---|
| TTTS (MC/DA) | Unbalanced arteriovenous anastomoses; fluid discordance | Fetoscopic laser photocoagulation for stages II–IV before ~26 weeks; beats serial amnioreduction |
| TAPS | Tiny anastomoses; large hemoglobin gap without fluid discordance | Detected by MCA peak systolic velocity (high in the anemic donor, low in the polycythemic recipient) |
| TRAP | Acardiac twin perfused backward through an artery-to-artery anastomosis; pump twin in high-output failure | Radiofrequency ablation of the acardiac twin's cord |
| Monoamniotic twins | No dividing membrane; cord entanglement nearly universal | Intensive surveillance (often inpatient from 24–28 weeks), steroids, cesarean at 32–34 weeks |
Selective reduction with intracardiac potassium chloride is never used in monochorionic twins: the drug crosses the shared circulation to the co-twin. Cord occlusion or ablation is used instead.
Timing and route of delivery
- Uncomplicated DC/DADeliver at 38 0/7–38 6/7 weeks.
- Uncomplicated MC/DADeliver at 34 0/7–37 6/7 weeks.
- MC/MACesarean at 32–34 weeks after antenatal steroids.
- ComplicationsFGR, TTTS, or preeclampsia move delivery earlier.
Route. When the presenting twin is cephalic, planned vaginal birth is as safe as planned cesarean, even if twin B is noncephalic; an experienced clinician can deliver twin B by breech extraction, internal podalic version, or external version. A breech or transverse twin A, monoamniotic twins, and conjoined twins are delivered by cesarean. Be ready for postpartum hemorrhage after any twin delivery.