What are the health considerations for monzygotic (monozygotic) twins?

Health Considerations for Monozygotic Twins

Monozygotic twins require intensive prenatal surveillance with chorionicity determination as the critical first step, as monochorionic twins face a 5-fold increase in fetal death and 7-fold increase in neonatal death compared to singletons, primarily from complications unique to shared placentation. 1

Chorionicity Determination and Timing

The timing of embryonic division after fertilization determines both placentation type and risk profile 1:

• Division within 0-4 days: Results in dichorionic-diamniotic twins (approximately one-third of monozygotic twins) with two separate placentas and lower risk profile 1
• Division at 4-8 days: Produces monochorionic-diamniotic twins (most common, representing two-thirds of monozygotic twins) with single shared placenta and 2 amniotic sacs—these face the highest complication rates 1
• Division at 8-12 days: Creates monochorionic-monoamniotic twins (<1% of cases) sharing both placenta and amniotic cavity with cord entanglement risk 1
• Division beyond 12-13 days: Results in conjoined twins (extremely rare) 1, 2

Critical First Trimester Assessment

Ultrasound determination of chorionicity must occur in the first trimester (ideally 10-14 weeks) using the "lambda sign" for dichorionic or "T sign" for monochorionic placentation. 1

Key early surveillance markers include 1:

• Crown-rump length (CRL) discrepancy ≥10%: Significantly associated with pregnancy loss and warrants specialist referral 1
• Nuchal translucency (NT) discordance ≥20%: Found in approximately 25% of monochorionic twins with up to 30% risk of severe TTTS or early fetal demise 1

Monochorionic-Specific Complications

Monochorionic twins account for 20% of twin pregnancies but 30% of all complications due to vascular anastomoses in the shared placenta 1:

Twin-Twin Transfusion Syndrome (TTTS)

• Overall mortality rate of approximately 10% in monochorionic-diamniotic pregnancies, primarily from TTTS and fetal anomalies 1
• Characterized by net flow from donor twin (oligohydramnios) to recipient twin (polyhydramnios) via arterio-venous connections 1
• Requires fetoscopic laser surgery for advanced cases 1

Twin Anemia-Polycythemia Sequence (TAPS)

• Occurs spontaneously in approximately 5% of monochorionic-diamniotic twins 1
• Middle cerebral artery Doppler surveillance should begin at 16 weeks with peak systolic velocity >1.5 multiples of median in donor and <1.0 in recipient 1

Twin Reversed Arterial Perfusion (TRAP) Sequence

• Rare complication where "pump" fetus perfuses anomalous acardiac mass 1

Single Fetal Demise Complications

• Vascular connections enable intravascular coagulation and embolization to surviving co-twin 3
• Can cause microcephaly, porencephalic cysts, hydranencephaly, intestinal atresia, aplasia cutis, and limb amputation 3
• Fetal MRI is helpful for assessing intracranial injury following spontaneous single fetal demise 1

Monochorionic-Monoamniotic Specific Risks

Survival rates exceed 90% with early diagnosis, serial ultrasound, and antenatal surveillance, though earlier studies suggested mortality rates of 46-64% 1:

• Entangled umbilical cords are typical and hallmark finding 1
• Preterm cesarean delivery is performed to avoid cord compromise and fetal demise 1
• Most deaths result from fetal malformations (including conjoined twins), TRAP sequence, TTTS, or preterm delivery before 20 weeks 1

Structural Anomalies and Genetic Considerations

Major fetal anomalies affect 1 in 25 dichorionic twins, 1 in 15 monochorionic-diamniotic, and 1 in 6 monoamniotic twin pregnancies 1:

• Early embryonic malformations (sirenomelia, holoprosencephaly, anencephaly) are increased in monozygotic twins 3
• Despite "identical" genetics, monozygotic twins show phenotypic differences due to chromosomal mosaicism, skewed X-inactivation, imprinting mechanisms, and epigenetic variations 4, 5
• Discordance for major malformations, growth, and lateral asymmetry is well-documented 5

Surveillance Protocol

Minimum ultrasound schedule for all twin pregnancies 1:

• First trimester dating scan
• Nuchal translucency scan at 11-14 weeks
• Anatomy scan at 18-22 weeks
• One or more third trimester growth scans

Monochorionic twins require more frequent surveillance influenced by complications and institutional practice 1:

• Every 2-week ultrasound monitoring for TTTS and growth discordance
• Middle cerebral artery Doppler beginning at 16 weeks for TAPS screening 1

Common Pitfalls to Avoid

• Failing to determine chorionicity in first trimester: The lambda sign becomes less reliable beyond first trimester as chorion frondosum regresses 1
• Assuming monozygotic twins are "genetically identical": Epigenetic and post-zygotic genetic differences exist and affect disease expression 4, 5
• Underestimating risk in dichorionic monozygotic twins: While lower risk than monochorionic, they still face higher anomaly rates than singletons 1