Enhancing prenatal risk assessment: atypicality Index (AcFTS) as an adjunct to the combined first trimester screening in dichorionic twins

To evaluate AcFTS as a multifactorial adjunct to the combined first trimester screening (cFTS) for enhancing prenatal risk assessment, pregnancy stratification and clinical decision-making in dichorionic (DC) twin pregnancies. A national Danish register-based cohort of all DC twin pregnancies with two viable fetuses at the cFTS, which is offered to all pregnant women in Denmark. Only pregnancies with Crown–rump lengths (CRL) between 45-84 mm in both fetuses, nuchal translucency thickness (NT) measurements, cFTS low-risk fetuses (< 1 in 100) of the major trisomies and known pregnancy outcome were included. The AcFTS were produced from details of NT, CRL, biparietal diameter, free β-human chorionic gonadotrophin, and pregnancy-associated plasma protein-A. The pregnancies were stratified into four groups according to their AcFTS. The 8,540 pregnancies included accounted for 96.1% of all DC twin pregnancies. Less typical (AcFTS(80-90]) and highly atypical (AcFTS≥99) pregnancies were associated with a significantly higher risk of single intrauterine fetal death (IUFD) than the overall. The risk of fetal reduction (FR) or birthweights of liveborn children classified as small for gestational age (SGA) or fetal growth restriction (FGR) was increased in the atypical (AcFTS(90-99]) and highly atypical pregnancies, compared to the overall. Typical pregnancies (AcFTS<80) had a significantly lower risk of FR. AcFTS is a clinically valuable tool for identifying DC twin pregnancies at risk of IUFD, FR, SGA or FGR despite being cFTS low-risk while providing reassurance for typical pregnancy profiles. 100% (8,540) Typical (AcFTS<80) 73.3% (6,262) Less typical (AcFTS(80-90]) 10.1% (862) Atypical (AcFTS(90-99]) 12.8% (1,089) Highly atypical (AcFTS≥99) 3.8% (327)