**2.7 Skeleton**

Detection of fetal anomalies is satisfactory for most organ systems but remains poor for cardiac, skeletal, and craniofacial anomalies. In a study to assess the accuracy of the prenatal diagnosis of skeletal dysplasias by Barbara V. Parilla et al., the antenatal diagnosis was correct in 20 (65%) of 31 cases [29]. This suggests that precise antenatal diagnosis of skeletal dysplasia is challenging. However, the antenatal prediction of lethality was highly accurate.

In general, skeletal dysplasias are uncommon. They affect 1 in every 4,000– 5,000 births, even though the incidence may be higher since the features may not be apparent until early childhood, at what time short stature, joint abnormalities or other complications become apparent.

Skeletal dysplasias comprise a heterogeneous group of disorders of skeletal growth that result in bones of abnormal size and shape.

Normally, all long bones are consistently seen from 11 weeks and the limbs move about readily at this gestation.

At the 18–23 week scan, the 3 segments of each limb should be visualized. It is nevertheless only necessary to measure the length of one femur. The relationship of the leg and foot should also be evaluated to rule out club foot.

US findings that are highly associated with the presence of a generalized skeletal dysplasia include shortening of extremity bones, fractures, bowing of long bones, demineralization, and a small thorax. A ratio of femur length to foot length of less than 0.9 and femur length–abdominal circumference ratio of less than 0.16 suggest a skeletal dysplasia.
