Multiple Gestation & Its Complications
A Step 2 CK–focused walkthrough of multiple gestation: chorionicity (not zygosity) determines risk, monochorionic placentation drives TTTS and related syndromes, and delivery timing and mode follow chorionicity plus the presenting twin's lie.
Overview: Chorionicity Drives Everything
Multiple gestation — twins, triplets, or higher-order — is a high-risk pregnancy with markedly increased rates of nearly every obstetric complication; incidence has risen with assisted reproduction and advanced maternal age. The board-critical distinction is zygosity (genetic origin) versus chorionicity (placentation), and chorionicity drives the risk. Dizygotic (fraternal) twins arise from two ova and are always dichorionic/diamniotic (DCDA). Monozygotic (identical) twins split from one zygote — the later the split, the fewer membranes shared and the higher the risk. Always determine chorionicity by ultrasound in the first trimester, when it is most accurate. Monochorionic twins share one placenta with vascular anastomoses, the anatomic substrate for twin-twin transfusion and its related syndromes.
- Chorionicity > zygosity for risk; assess by ultrasound before 14 weeks
- Lambda / twin-peak sign = dichorionic; T sign = monochorionic
- Dizygotic = always DCDA; monozygotic chorionicity depends on timing of the split
- Suspect twins: uterine size > dates, exaggerated hCG, elevated MSAFP, severe hyperemesis gravidarum
- Preterm birth is the leading cause of perinatal morbidity and mortality
- Maternal risks all rise: preeclampsia, gestational diabetes, iron/folate anemia, postpartum hemorrhage (uterine atony), cesarean delivery
- Low-dose aspirin (81 mg, start 12–28 wk, ideally <16 wk): multifetal gestation is a high-risk factor for preeclampsia
- Monochorionic-only: TTTS, TAPS, selective IUGR, co-twin demise injury; MCMA adds cord entanglement
Split Timing → Placentation
| Split (post-fertilization) | Chorionicity/Amnionicity | Frequency (of MZ) | Ultrasound clue |
|---|---|---|---|
| Days 0–3 (morula) | Di/Di (DCDA) | ~25–30% | Lambda / twin-peak |
| Days 4–8 (blastocyst) | Mono/Di (MCDA) | ~70% (most common MZ) | T sign |
| Days 8–13 | Mono/Mono (MCMA) | ~1–2% | Single sac, no dividing membrane |
| >13 days | Conjoined twins | rare | Fused fetuses |
Vignette: At 22 weeks, ultrasound of monochorionic-diamniotic twins shows one small twin with oligohydramnios, pinned against the uterine wall (the "stuck twin"), and one large twin with polyhydramnios and a distended bladder.
- Diagnosis: Twin-twin transfusion syndrome (TTTS) — unbalanced net flow across placental vascular anastomoses. Donor: oliguria, oligohydramnios, growth restriction, anemia. Recipient: polyhydramnios, polycythemia, volume overload → cardiomegaly/hydrops.
- Occurs only in monochorionic pregnancies (shared placenta).
- Next / definitive step (Quintero stage ≥ II, before 26 weeks): fetoscopic laser photocoagulation of the anastomoses. Serial amnioreduction is an alternative/adjunct for symptomatic polyhydramnios.
TTTS — "Donor is Dry, Recipient is Rich":
- Donor = Dry: oligohydramnios, oliguria, small, anemic — the "stuck twin"
- Recipient = Rich / Ruddy: polyhydramnios, polycythemia (plethoric), volume-overloaded → hydrops
Chorionicity signs:
- λ (Lambda) has Layers → diChorionic — thick chorionic twin-peak
- T is Thin / Together → Monochorionic — membranes meet the placenta at ~90°
Vignette: DCDA twins at 38 weeks in active labor. Twin A (presenting) is vertex; Twin B is breech.
- Presenting Twin A vertex → attempt vaginal delivery.
- Twin B non-vertex (after Twin A delivers): acceptable options are breech extraction / internal podalic version or cesarean, guided by operator experience and estimated fetal weight.
- If Twin A (presenting) is non-vertex → cesarean delivery.
- MCMA (mono-mono) twins: cord-entanglement risk mandates scheduled cesarean at 32–34 weeks with intensive inpatient fetal monitoring.
Recommended Delivery (Uncomplicated)
| Type | Deliver at | Mode |
|---|---|---|
| DCDA | 38 0/7–38 6/7 weeks | Vaginal if Twin A vertex |
| MCDA | 34 0/7–37 6/7 weeks (~36) | Vaginal if Twin A vertex |
| MCMA | 32 0/7–34 0/7 weeks | Scheduled cesarean |
| Presenting twin non-vertex | per indication | Cesarean |
- Single fetal demise in monochorionic: acute hypotension through anastomoses → co-twin death or neurologic injury (ischemic, not merely thromboembolic)
- Growth discordance significant when estimated-weight difference ≥ 20%
- TAPS (twin anemia-polycythemia sequence): chronic slow transfusion → donor anemia + recipient polycythemia without amniotic-fluid discordance; diagnosed by MCA-PSV Doppler discordance
- TRAP sequence (acardiac twin): normal "pump" twin perfuses an acardiac mass via reversed perfusion → high-output heart failure
- Vanishing twin: early first-trimester loss of one twin
- Higher congenital anomalies in monozygotic twins
- Also increased: PPROM, malpresentation, cord prolapse, placenta previa, abruption
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