Monochorionic diamniotic twin gestations discordant for markedly enlarged nuchal translucency

To assess karyotypes and outcomes of monochorionic diamniotic (MCDA) twin pregnancies discordant for markedly enlarged nuchal translucency (NT) in the first trimester.

Brigham and Women's Hospital's ultrasound database was queried to identify all MCDA gestations in which one twin had NT ≥ 3.5 mm and the co-twin had normal NT. Cytogenetic results, ultrasound findings, and pregnancy outcomes were reviewed.

Of 162 MCDA twin pairs, 11 were discordant for NT ≥ 3.5. Chromosomal abnormalities were present in three cases: one twin pair was concordant for trisomy 18; one pair discordant for mosaic trisomy 2; and one pair discordant for confined placental mosaicism (CPM) (high frequency tetraploidy). Adverse outcomes for twins with euploid or unknown karyotypes included twin reverse arterial perfusion (TRAP) sequence, growth discordance, and esophageal atresia with tracheoesophageal fistula.

Postfertilization nondisjunction leading to mosaicism in one fetus, discordant phenotypes yet concordant karyotypes, and discordance for CPM were documented phenomena, supporting karyotyping of both twins. In this case series, discordant NT was a marker for chromosome abnormalities, as well as for complications specific to monochorionic gestations, including TRAP sequence, amniotic fluid discordance, and structural anomalies. Nonetheless, normal fetal anatomy and karyotype were the most common outcomes.