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].