ABSTRACT
Shortened femur length is the most common presentation for suspected skeletal problems as this is the only bone routinely measured in obstetric ultrasound. In the majority of cases, shortening of the long bones is likely to represent inaccurate dating, constitutional smallness, or an early feature of fetal growth restriction. Unilateral femoral shortening is suggestive of focal femoral hypoplasia syndromes. Typically, the prognosis is good in most of these syndromes. The finding of bilateral shortening is suggestive of achondrogenesis. Bowing may be difficult to differentiate from fractures of the long bones, but both suggest the diagnoses of achondrogenesis, thanatophoric dysplasia, campomelic dysplasia, or osteogenesis imperfecta. Definitive hypomineralization is indicative of either hypophosphatasia or osteogenesis imperfecta. Polydactyly is characteristic of short-ribbed polydactyly at this gestation and microcephaly occurs in chondrodysplasia punctata. The most common diagnosis in the third trimester is achondroplasia. Spondyloepiphyseal dysplasias congenital is an alternative diagnosis, but this is rarely made pre-natally as the ultrasound features are subtle.
