In a study published in Ultrasound in Obstetrics & Gynecology that involved more than 50,000 pregnancies, a fetal anomaly was detected for the first time in the third trimester in one in 200 women who had undergone a first and/or second trimester ultrasound examination.
Most of the fetal abnormalities (68%) seen at 35 to 37 weeks had already been diagnosed in the first and/or second trimester, and the incidence of abnormalities first seen at 35 and 37 weeks was 0.5%.
The study highlights the benefit of a late third trimester scan for detecting fetal abnormalities that were either missed in previous first and second trimester scans or became apparent only during the third trimester.
Ficara, A., et al. (2019) Value of routine ultrasound examination at 35–37 weeks' gestation in diagnosis of fetal abnormalities. Ultrasound in Obstetrics & Gynecology. doi.org/10.1002/uog.20857.