Which classic ultrasound finding is specific to trisomy 13?

The identification of midline craniofacial abnormalities is a classic ultrasound finding that is specifically associated with trisomy 13, also known as Patau syndrome. This condition is characterized by a wide range of severe developmental anomalies, and craniofacial features such as cleft lip and/or palate, holoprosencephaly, and severe midline facial defects are particularly indicative of this genetic disorder.

Midline abnormalities stem from disruptions during early embryonic development, particularly involving the forebrain, which leads to the characteristic facial deformities seen in affected individuals. Not only can these defects be detected through prenatal ultrasound, but they also highlight the abnormal early migration of neural crest cells.

In contrast, while cardiac defects, short femur length, and spinal defects can be associated with a variety of chromosomal abnormalities, they are not specific to trisomy 13. Cardiac defects are common in many different syndromes; short femur length could indicate several factors, including growth restriction or other aneuploidies; and spinal defects can occur due to a range of genetic or environmental factors. Therefore, midline craniofacial abnormalities remain a defining characteristic that sets trisomy 13 apart from other conditions.