Undetected cases after implementation of first-trimester anomaly scan in low-risk population: insights from the IMITAS study.

OBJECTIVE: To assess the effectiveness of the first-trimester anomaly scan (FTAS) performed as part of a centrally steered national screening program in The Netherlands by investigating false-negative cases with a fetal structural anomaly that was not detected at the FTAS. METHODS: This was a secondary analysis of the IMplementation of fIrst Trimester Anomaly Scan (IMITAS) study, a national prospective cohort study conducted in a low-risk population in The Netherlands between November 2021 and November 2022. The FTAS was performed according to a predefined scanning protocol. We collected all cases with a fetal structural anomaly that was not detected at the FTAS and was diagnosed at a tertiary care center before 24 weeks' gestation following a referral based on the second-trimester anomaly scan. These anomalies were classified as: (1) 'always detectable' in the first trimester (e.g. holoprosencephaly, anencephaly), referred to as first-trimester major anomalies (FTMAs); (2) 'often detectable' in the first trimester (e.g. spina bifida, major heart defect); or (3) 'undetectable' in the first trimester (e.g. agenesis of the corpus callosum). To investigate the reasons for undetected FTMA, sonographic images from these cases and from a sample of normal controls were retrieved and scored for quality by two fetal medicine experts, who were blinded to the fetal outcome. RESULTS: Of the 127 979 cases with a normal FTAS result, 1164 (0.9%) had a fetal structural anomaly diagnosed before 24 weeks' gestation. Seven cases could not be categorized because intrauterine fetal death occurred before arrival at the tertiary care center. Among the 1157 remaining cases with an undetected anomaly, 23 (2.0%) had a FTMA, comprising mainly limb reduction defects, hydrops and megacystis, with seven cases of multiple congenital anomalies. A further 126 (10.9%) cases had an anomaly that is considered often detectable in the first trimester, which primarily comprised cardiac defects and central nervous system anomalies. Lastly, 1008 (87.1%) cases had an anomaly that is considered undetectable in the first trimester, represented mainly by urogenital and cardiac anomalies. Quality assessment of images obtained at the FTAS in 23 cases with undetected FTMA and 19 controls showed that the quality score for fetal anatomy was significantly lower in cases vs controls (P = 0.003), with mandatory planes missing more often (P = 0.039). CONCLUSIONS: Nearly 1% of pregnancies with a normal FTAS result were diagnosed with a structural anomaly in the second trimester, including 23 major anomalies amenable to detection in the first trimester (accounting for 0.02% of scans with a normal result). Missing mandatory planes and acquisition of suboptimal planes were the main reasons for the overlooked FTMA diagnoses. Improvements are expected with increasing sonographer experience, additional training and possible expansion of the protocol. © 2025 The Author(s). Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.