Timing of diagnosis of fetal structural abnormalities after the introduction of universal cell‐free DNA in the absence of first‐trimester anatomical screening

INTRODUCTION: Since 2021, first‐trimester anatomical screening (FTAS) is offered in the Netherlands alongside genome‐wide cell‐free DNA (cfDNA). Previously, only second‐trimester anatomical screening (STAS) was offered. This study identifies structural abnormalities amenable to first‐trimester diagnosis detected at/after STAS in the period following cfDNA implementation and preceding FTAS introduction. METHODS: This retrospective cohort includes 547 fetuses referred between 2017 and 2020 because of suspected structural abnormalities before/at/after STAS. Additional prenatal investigations and postnatal follow‐up were searched. Abnormalities were classified into “always”, “sometimes”, and “never” detectable in the first‐trimester based on a previously suggested classification. RESULTS: Of the 547 pregnancies, 13 (2.6%) received FTAS and 534 (97.6%) received a dating ultrasound and STAS. In 492/534 (92.1%) anomalies were confirmed; 66 (13.4%) belonged to the “always detectable” group in the first trimester, 303 (61.6%) to the “sometimes detectable”, and 123 (25.0%) to the never detectable. Of the “always detectable” anomalies 29/66 (44%) were diagnosed during dating ultrasounds and 37 (56%) during STAS. The rate of termination of pregnancy for anomalies detected during FTAS and at/after STAS was 84.6% (n = 11/13) and 29.3% (n = 144/492) (p < 0.01). CONCLUSION: When FTAS is not part of screening paradigms, most fetal anomalies remain undetected until the second trimester or later in pregnancy, including 56% of anomalies “always detectable” in the first trimester.