Pregnancy with >=2 fetuses; chorionicity drives risk profile and management.
Also known as: twins, multiple gestation, twin pregnancy, triplet pregnancy, MCDA, DCDA
Overview
Pregnancy carrying two or more fetuses. Classified by zygosity (monozygotic vs dizygotic) and, more clinically importantly, by chorionicity and amnionicity: dichorionic-diamniotic (DCDA), monochorionic-diamniotic (MCDA), or monochorionic-monoamniotic (MCMA).
Epidemiology
Twin rate ~3% of live births (driven up by assisted reproductive technology and advanced maternal age). About two-thirds of spontaneous twins are dizygotic.
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Assisted reproductive technology (especially IVF with multiple embryo transfer, ovulation induction with clomiphene or letrozole or gonadotropins)
Advanced maternal age (higher FSH levels)
Family history of dizygotic twins (maternal side)
African ancestry (higher dizygotic rate); Asian ancestry (lower)
Higher parity, taller stature
Pathophysiology
Dizygotic twins arise from fertilization of two separate ova by two sperm — always DCDA, separate placentas, genetically siblings. Monozygotic twins arise from a single fertilized egg splitting. Timing of cleavage determines chorionicity: 0-3 days = DCDA; 4-8 days = MCDA (most common monozygotic, ~70%); 8-13 days = MCMA; >13 days = conjoined twins.
Clinical presentation
Symptoms
Often diagnosed on first-trimester ultrasound
Exaggerated symptoms of pregnancy (nausea, breast tenderness)
Earlier and larger fundal height for gestational age
Earlier perception of fetal movement
Signs / physical exam
Fundal height greater than expected (>3 cm above expected for GA)
Auscultation of two distinct fetal heart rates with different rhythms
Maternal weight gain above expected
Differential diagnosis
Singleton with hyperestrogenic uterine enlargement / fibroids — Single fetal pole on US; fibroids visualized
Molar pregnancy — Very high beta-hCG, no fetus, 'snowstorm' US — rare combined molar + viable twin requires expert management
Diagnostic workup
Diagnostic criteria
Ultrasound visualization of multiple gestational sacs/embryos with documentation of chorionicity in the first trimester.
Labs
Routine prenatal labs as for singleton; CBC follow-up given higher anemia risk
First-trimester aneuploidy screening with cell-free DNA is more complex in twins (must specify dichorionic vs monochorionic for risk interpretation)
Glucose tolerance testing (higher GDM rates)
Imaging
First-trimester ultrasound (6-14 wk) — most accurate time to determine chorionicity and amnionicity
Serial growth ultrasounds every 4 weeks (dichorionic) or every 2 weeks starting at 16 wk (monochorionic)
Targeted anatomic survey at 18-22 wk
Antepartum surveillance (NST/BPP) per chorionicity and complications
Diagnostic algorithm
Type
Chorionicity/Amnionicity
First-Trimester US Sign
Recommended Delivery
Dichorionic-Diamniotic
2 placentas, 2 sacs
Lambda / twin peak sign
38 0/7 - 38 6/7 wk
Monochorionic-Diamniotic
1 placenta, 2 sacs
T sign
36 0/7 - 37 6/7 wk
Monochorionic-Monoamniotic
1 placenta, 1 sac
No dividing membrane
32 0/7 - 34 0/7 wk by cesarean
Twin classification, ultrasound signs, and ACOG/SMFM recommended delivery timing.
Treatment
First-line
Early establishment of chorionicity is essential — drives surveillance plan
Folic acid 1 mg daily (some recommend higher in multiples)
Nutritional counseling: greater caloric and weight gain targets per IOM
Low-dose aspirin 81 mg daily starting 12-28 wk, ideally before 16 wk (USPSTF/ACOG/SMFM — multiple gestation is a HIGH-risk factor for preeclampsia; a single high-risk factor alone warrants aspirin)
Discourage routine bed rest, prophylactic tocolysis, and prophylactic cerclage — none reduce preterm birth
Screen for gestational diabetes and anemia; iron supplementation
DCDA — delivery timing
Recommended delivery 38 0/7 to 38 6/7 wk per ACOG/SMFM if uncomplicated
MCDA — delivery timing and monitoring
Serial growth and amniotic fluid ultrasounds every 2 weeks from 16 wk to monitor for twin-twin transfusion syndrome (TTTS), selective IUGR, twin anemia-polycythemia sequence (TAPS)
Recommended delivery 36 0/7 to 37 6/7 wk if uncomplicated
MCMA — delivery timing and monitoring
Inpatient monitoring usually from 24-28 wk due to cord entanglement risk
Recommended delivery 32 0/7 to 34 0/7 wk by cesarean
Mode of delivery (DCDA/MCDA)
Vaginal delivery acceptable when both twins vertex; consider for vertex/non-vertex if experienced operator (breech extraction of second twin)
Cesarean for non-vertex presenting twin, monoamniotic twins, or conjoined twins
Second-line / adjunct
Fetoscopic laser photocoagulation for severe TTTS at 16-26 wk (Solomon technique)
Selective reduction for higher-order multiples (typically triplet to twin) to improve outcomes
Complications
Preterm birth (>50% of twins, ~90% of triplets)
Hypertensive disorders of pregnancy (preeclampsia rate doubled), gestational diabetes, anemia
Operative delivery, postpartum hemorrhage, increased NICU admission and neonatal mortality
PANCE pearls
Chorionicity is most accurately determined in the first trimester — request and document this on any twin ultrasound.
TTTS (oligohydramnios in donor, polyhydramnios in recipient with intertwin discordance) is treated with fetoscopic laser ablation, NOT amnioreduction alone, when criteria met.
Monoamniotic twins are at risk of cord entanglement and stillbirth — inpatient monitoring with daily NST is standard from 24-28 wk.
Vanishing twin: loss of one twin in the first trimester occurs in up to 30% of twins; usually no maternal sequelae.
Twin B is at higher risk of cord prolapse, abruption, and intrapartum compromise once Twin A delivers.
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