Routine 36-week scan: diagnosis of fetal abnormalities

To investigate further the incidence and types of fetal abnormality identified at a routine 36-week ultrasound examination, which had not been diagnosed in previous scans at 20 weeks and 12 weeks' gestation, and to report the fetal abnormalities that are diagnosed only postnatally.

This was a prospective study of 104 151 women with a singleton pregnancy attending for a routine ultrasound examination at 35 + 0 to 36 + 6 weeks' gestation. In each case, a detailed examination was carried out for the diagnosis of fetal abnormality. All women had undergone a previous ultrasound examination at 19 + 0 to 23 + 6 weeks and 95 801 (92.0%) women also had a scan at 11 + 0 to 14 + 1 weeks. We excluded pregnancies with known aneuploidy. Fetal abnormalities were classified according to the affected major organ system, and the type and incidence of new abnormalities were determined.

There were four main findings of this study. First, in the study population, abnormality was identified in 2552 (2.5%) fetuses/neonates. Second, at the 36-week scan, abnormality was detected in 2144 (2.1%) fetuses and the most common abnormalities first detected at the 36-week scan were ventricular septal defect, unilateral or bilateral hydronephrosis, unilateral empty renal fossa (with or without pelvic kidney), unilateral or bilateral duplex kidney and mild ventriculomegaly. Third, 1341 (62.5%) of the fetuses with abnormality detected at the 36-week scan had been diagnosed previously during the first or second trimester and therefore, the incidence of abnormality detected for the first time in the third trimester was 0.77% (803/104 151). The most common abnormalities that were diagnosed exclusively for the first time during the third-trimester scan were ovarian cyst, achondroplasia, microcephaly, vein of Galen malformation and hematocolpos. Fourth, the incidence of abnormality detected for the first time postnatally was 0.39% (408/104 151). The most common abnormalities detected for the first time postnatally were polydactyly, oligodactyly or syndactyly, hypospadias/epispadias, mild talipes treated with physiotherapy, ventricular septal defect and isolated cleft palate. The most common abnormalities diagnosed exclusively for the first time postnatally were isolated cleft palate, anal atresia, atrial septal defect and esophageal atresia with fistula.

A high proportion of fetal abnormalities are detected for the first time during a routine 36-week scan. Such diagnosis and subsequent management, including the selection of timing and place for delivery and postnatal investigation, could potentially improve postnatal outcome. © 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.