Routine ultrasound examination at 35–37 weeks' gestation may reveal new fetal abnormalities that could not be diagnosed at earlier examinations.
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 (68%) of the fetal abnormalities seen at 35 to 37 weeks' gestation had already been diagnosed in the first and/or second trimester. The incidence of abnormalities first seen at 35–37 weeks was 0.5%, and those that were detected exclusively for the first time at this examination were ovarian cyst, microcephaly, achondroplasia, dacryocystocele and hematocolpos.
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. Such diagnosis and subsequent management, including selection of timing and place for delivery and postnatal investigations, could potentially improve postnatal outcome.