Can Ultrasound Detect Fetal Anomalies?

Ultrasound imaging—commonly referred to as “B-mode” or “B-scan” ultrasound—is a cornerstone of prenatal screening and plays a critical role in the detection of fetal structural anomalies. When performed by experienced sonographers or maternal-fetal medicine specialists using high-resolution equipment, routine obstetric ultrasound between 18 and 24 weeks’ gestation can identify a wide range of major congenital malformations, including but not limited to neural tube defects (e.g., anencephaly, spina bifida), major cardiac anomalies, cleft lip and palate, abdominal wall defects (e.g., gastroschisis, omphalocele), renal agenesis or severe hydronephrosis, and limb-reduction deformities.

However, it is essential to recognize that ultrasound is a screening—not a diagnostic—tool for many conditions. Its sensitivity varies significantly depending on fetal position, maternal body habitus, amniotic fluid volume, gestational age, and the specific anomaly under consideration. For example, while open spina bifida may be detected with >90% sensitivity in optimal conditions, subtle cardiac defects or certain genetic syndromes without overt structural markers often remain undiagnosed prenatally. Additionally, some abnormalities—such as mild ventriculomegaly or isolated echogenic intracardiac foci—may be transient findings with no clinical significance, underscoring the importance of expert interpretation and, when indicated, confirmatory testing (e.g., fetal echocardiography, amniocentesis with chromosomal microarray).

Current guidelines from professional societies—including the American College of Obstetricians and Gynecologists (ACOG) and the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG)—recommend a detailed anatomy scan during the late second trimester as part of standard prenatal care. While this examination substantially enhances early identification of serious structural birth defects, clinicians must counsel patients transparently: ultrasound cannot guarantee the detection of all fetal anomalies, and a normal scan does not rule out every possible condition—including functional disorders, metabolic diseases, or neurodevelopmental conditions that manifest postnatally.