Dedicated First-Trimester Anomaly Scan in a National Prenatal Screening Program and Timing of Diagnosis: The Prospective IMITAS Cohort Study

Objective: To determine the diagnostic yield of a nationally implemented first-trimester anomaly scan (FTAS), compared to standard obstetric care with a second-trimester anomaly scan. Design: Prospective observational cohort study. Setting: Nationwide prenatal screening program in the Netherlands. Population: Pregnant women referred for suspected fetal anomalies with subsequent abnormal diagnostic scans in 2021–2022. Methods: All fetal diagnoses < 18 weeks' gestational age (GA) were registered: 8 months before (BEFORE) and 1 year after (AFTER) FTAS implementation. BEFORE-cohort: referrals after routine scans (e.g., dating scans). AFTER-cohort: all referrals, including FTAS. Corrections were made for missing referrals < 12 + 3 weeks in the AFTER-cohort. Time adjustment was made in the BEFORE-cohort. Anomalies were categorised as: First-Trimester Major Anomalies (FTMA), often detectable anomalies and other anomalies. Main Outcome Measures: Number of fetal anomalies, time to final diagnosis, GA at termination of pregnancy (TOP). Results: Abnormal diagnostic scans increased from 750 to 1261 (BEFORE vs. AFTER) with definitive diagnoses in 609 vs. 940. Time to diagnosis and GA at TOP in c-BEFORE were comparable to c-AFTER (11 vs. 13 days and GA 14 + 6 vs. 15 + 1, respectively). FTAS increased the detection of often detectable and other anomalies (53 vs. 124 and 248 vs. 474, respectively). FTMA slightly increased (308 vs. 342). Conclusions: FTAS substantially increases detection of fetal anomalies, primarily often detectable and other anomalies. Overall GA at diagnosis or TOP increased marginally, but some cases required an extended period of diagnostic evaluation.