In monochorionic diamniotic (MoDi) twins, what is the most associated marker with aneuploidy?

Thickened Nuchal Translucency Is Most Associated with Aneuploidy in Monochorionic Diamniotic Twins

Among the options presented, thickened nuchal translucency is most strongly associated with aneuploidy in monochorionic diamniotic (MoDi) twins. 1

Evidence Supporting Nuchal Translucency as Primary Marker

Nuchal translucency (NT) serves as a powerful and independent ultrasound marker for fetal aneuploidy in twin pregnancies. The American College of Radiology (ACR) guidelines specifically highlight that:

• Increased NT in monochorionic twins is associated with fetal aneuploidy, genetic syndromes, structural anomalies, and intrauterine demise 1
• First trimester combined screening with NT and maternal serum markers is a common choice for aneuploidy screening in monochorionic twins 1
• A meta-analysis of first trimester combined aneuploidy screening reported a sensitivity of 87.4% for monochorionic twins 1

Comparison with Other Markers

When comparing the markers mentioned in the question:

1. Thickened nuchal translucency: Primary marker with established sensitivity of 87.4% for detecting aneuploidy in monochorionic twins 1

2. Absent nasal bone: While the absence of nasal bone is mentioned as a helpful additional marker, it's described as "more challenging in twin pregnancies" 1 and is considered supplementary to NT measurement rather than the primary marker.

3. Echogenic bowel: Not specifically highlighted as a major marker for aneuploidy in monochorionic twins in the provided evidence.

4. Reverse flow in ductus venosus: While reversed a-waves in ductus venosus are mentioned as markers for congenital heart disease and twin-twin transfusion syndrome, they are not emphasized as primary markers for aneuploidy in monochorionic twins 1.

Clinical Application in MoDi Twins

In monochorionic twins:

• Each fetus is assumed to have the same risk of aneuploidy equivalent to the maternal age risk of a singleton 1
• Aneuploidy risk estimates for trisomies 21,18, and 13 are calculated using the mean NT measurement of the twins 1
• NT measurements are observed to be higher in monochorionic twins compared to dichorionic twins, which explains the higher false-positive rate 1

Case Evidence

Research evidence supports the association between increased NT and chromosomal abnormalities in monochorionic twins:

• A case report documented monochorionic diamniotic twins discordant for markedly enlarged NT (7.7 mm) in one fetus, which was found to have monosomy 21 2
• Another study of monochorionic diamniotic twin gestations discordant for markedly enlarged NT found that discordant NT was a marker for chromosome abnormalities 3

Important Caveats

• While NT is the strongest marker, it has limitations with a higher false-positive rate in monochorionic twins compared to singletons 1
• Assessment for additional first trimester markers (like absent nasal bone) may be helpful in cases with increased NT to refine risk assessment 1
• The incidence of chromosomal abnormalities is actually higher in dichorionic twins (25.4%) than in monochorionic diamniotic twins (15.3%) 4, but NT remains the most reliable marker for detection in MoDi twins

Clinical Approach

For monochorionic diamniotic twins with suspected aneuploidy:

1. Measure NT in both fetuses at 11-14 weeks gestation
2. Calculate the mean NT for risk assessment
3. Consider additional markers (nasal bone, ductus venosus flow) if NT is increased
4. Offer genetic counseling and invasive diagnostic testing when indicated
5. Follow with detailed anatomic survey and fetal echocardiography if pregnancy continues with increased NT

Remember that increased NT in monochorionic twins may also indicate other complications like twin-twin transfusion syndrome, so comprehensive evaluation is essential.