NT measurement
NT ultrasound was performed by physicians between 11 and 14 gestational weeks (fetal crown–rump length (CRL) between 45mm and 84 mm) in each institution. Fetal NT was measured according to the protocol recommended by Fetal Medicine Foundation [9]. Increased NT was diagnosed as 95th percentile or more [3].The 99th percentile was defined as NT ≥ 3.5 mm for all gestational ages, while the 95th percentile was adjusted for gestational age [10].