DNB Answer: Zygosity, Chorionicity, and Management of MCMA Twin Pregnancy (10 Marks: 3+3+4)

Part 1 - Zygosity of Twin Pregnancy (3 marks)

Types:

• Arise from division of a single fertilized ovum (one sperm + one ovum)
• Share approximately 100% of their genetic material
• Occur at a constant rate of ~3-4 per 1000 births worldwide
• The timing of zygotic division determines the type of placentation (chorionicity):

• Arise from fertilization of two separate ova by two separate sperm
• Share ~50% of genes (same as non-twin siblings)
• Show large regional variation: 6/1000 births in Asia to 40/1000 births in sub-Saharan Africa
• Always result in dichorionic-diamniotic placentation (rarely, dizygotic monochorionic twins occur with ART)
• Influenced by maternal age, race, parity, family history, ART

Clinical Determination of Zygosity:

• Antenatal: Ultrasound (fetal sex discordance = dizygotic; same sex = may be mono or dizygotic)
• Postnatal: Placental examination, blood group testing (discordant blood groups = proof of dizygosity), DNA microsatellite marker analysis (gold standard)

Part 2 - Chorionicity of Twin Pregnancy (3 marks)

Types:

• Two separate chorions and two amnions
• Can have separate placentas or a single fused placenta (with separate circulations)
• All DZ twins + ~30% MZ twins
• Lower risk pregnancy

• Single shared chorion, two amnions (dividing membrane present)
• All are MZ (or rarely DZ with ART)
• At risk for: TTTS, TAPS, TRAP sequence, growth discordance
• Perinatal mortality ~12% vs 2-5% for DCDA

• Single chorion, single amnion, no dividing membrane
• Highest-risk category; historically perinatal mortality >50%
• At risk for: cord entanglement, TTTS, TAPS, congenital anomalies

Determination of Chorionicity:

• Lambda/"Twin-peak" sign - triangular wedge of echogenic trophoblastic tissue projecting into the base of the inter-twin membrane = Dichorionic
• "T" sign - dividing membrane meets the placenta at 90° without a trophoblastic projection = Monochorionic
• Absent dividing membrane = Monoamniotic
• Sex discordance = Dichorionic
• Two clearly separate placental sites = Dichorionic
• Thick (4-layer), multilayered dividing membrane = Dichorionic

Clinical importance: Chorionicity, not zygosity, determines obstetric risk and management. Monochorionic twins require more intensive surveillance.

Part 3 - Management of Monochorionic-Monoamniotic (MCMA) Twin Pregnancy (4 marks)

Definition & Background

Diagnosis

• Ultrasound: No inter-twin dividing membrane identified by an experienced sonographer
• Color Doppler: Entangled umbilical cords (pathognomonic; detectable from ~10 weeks)
• Pulsed Doppler: Two distinct heart rates within the same Doppler gate
• Single yolk sac before 10 weeks (less specific; needs follow-up)

Antenatal Management

• Refer to a Fetal Medicine Unit
• Counsel regarding high fetal loss rate, prematurity, and cord entanglement risk
• Option of termination of pregnancy if diagnosed in early 1st trimester

• Detailed structural ultrasound at 18-20 weeks (high rate of congenital anomalies)
• Fetal echocardiography
• Chromosomal assessment if indicated

• Fortnightly growth scans + Doppler from 16 weeks (to detect TTTS/TAPS/growth discordance)
• Daily non-stress tests (NSTs) starting at 24-26 weeks' gestation - look for variable decelerations (indicator of cord compression)
• If variable decelerations increase in frequency: continuous fetal heart rate monitoring with readiness for emergency cesarean delivery
• Biophysical profile (BPP) as adjunct

• The MONOMONO study (multicenter cohort, 195 women) found standard regression analysis showed inpatient management from 25 weeks was associated with significantly lower fetal demise rates (3.3% vs 10.8%) and lower perinatal death rates (4% vs 15%) compared to outpatient management
• Many centers therefore recommend hospital admission from 24-26 weeks for continuous or near-continuous monitoring
• The benefit of inpatient care remains debated; further trials are needed

• Administer a course of betamethasone prior to elective preterm delivery (at 32-34 weeks)

• Has been proposed to reduce amniotic fluid volume, potentially stabilizing fetal lie and reducing cord entanglement risk
• No current evidence to support its routine use; not recommended

Timing and Mode of Delivery

• Elective cesarean delivery at 32-34 weeks' gestation is the standard of care at most centers
• ACOG recommends delivery by 34 weeks
• No fetal or neonatal deaths were seen in one series beyond 32 weeks, but a subsequent case of double fetal death at 35 weeks cautioned against expectant management
• Japanese cohort data showed a 5-8% prospective risk of fetal death per week between 30-36 weeks
• Balancing act: prematurity-related RDS (~5% incidence with surfactant) vs. ongoing stillbirth risk
• Pre-delivery corticosteroids improve fetal lung maturity

• Cesarean section is strongly preferred - most centers do not offer vaginal delivery for viable MCMA twins
• Vaginal delivery risk: tangled cords may tighten during expulsion; there are case reports of cutting the nuchal cord of the second twin after delivery of the first twin's head
• Emergency cesarean is performed if fetal heart rate testing becomes non-reassuring at any gestational age after viability

Summary Table: MCMA Management

Complications to Monitor

• Cord entanglement (primary risk)
• Twin-to-twin transfusion syndrome (TTTS)
• Twin anemia-polycythemia sequence (TAPS)
• TRAP (acardiac twin) sequence
• Discordant fetal growth / IUGR
• Structural anomalies (higher rate in MZ twins)
• Preterm birth