The prenatal period is a period of dramatic change in the size and shape of the proximal femur. Following limb formation, from 12 to 40 weeks of gestation femoral head diameter increases more than four-fold, whilst femoral torsion and inclination increase by around 40° and 7.5° respectively. The relative magnitude of change in femoral head size over this prenatal period is similar to that experienced in the first two decades of postnatal life, after which little change is observed

. One factor thought to contribute substantially to prenatal joint development is mechanical loading caused by reaction and muscle forces during fetal movements such as kicking. The importance of these movements for skeletal development can be seen in rare cases of neuromuscular conditions of fetal onset, which lead to slender, fracture-prone bones.

In addition, a number of less severe prenatal conditions have been identified which lead to restrictions in movement and result in lower stresses applied to bones.

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