Combining Detailed Fetal Anatomy Scanning in the NT Window Versus Early Second Trimester Scanning at 14-16 Weeks: A Non-Inferiority Study.

Abstract

Objectives: First-trimester ultrasound has evolved to incorporate a detailed fetal anatomy scan (FAS) with nuchal translucency (NT) screening. Many institutions use a 2-visit protocol: NT followed by detailed FAS at 14-16 weeks. We aimed to evaluate whether integrating detailed FAS into the NT window (12 + 5 to 13 + 6 weeks) is non-inferior in diagnostic yield to the 2-visit protocol.

Methods: We enrolled 755 mixed high- and low-risk pregnant women with singleton gestations into either integrated NT + FAS group (n = 243, 12 + 5 to 13 + 6 weeks) or 2-visit group (n = 512, 14-16 weeks). All underwent follow-up at 20-24 weeks. Scans followed ISUOG guidelines; the primary outcome was detection of fetal anomalies during detailed early scans. Non-inferiority was defined as a margin of ≤6% difference in detection rates.

Results: Study groups were similar except in parity (mean 2.6 [0-9] vs. 3.1 [0-10], P = .0081 in FAS + NT and 2-visit group, respectively). Anomalies were identified in 16 fetuses (7.5%) in the NT + FAS group and 16 (3%) in the 2-visit group (P = .033). Additional anomalies were detected at mid-trimester in 2/227 (0.9%) NT + FAS and 4/496 (0.8%) 2-visit group fetuses (P = 1.0). The integrated approach met non-inferiority criteria.

Conclusion: Detailed FAS during the final NT window week is non-inferior to the 2-visit approach. This integrated protocol offers earlier reassurance without compromising diagnostic accuracy.