Congenital femoral deficiency (CFD) refers to a spectrum of congenital (present at birth) malformations of the thigh bone (femur) due to incomplete or abnormal development. CFD may affect one leg (most commonly) or both legs. Severity can range from minor shortening of the femur (appearing normal), to complete absence of much or all of the femur. Deficiency or instability of the hip and knee joint may also be also present. The underlying cause of CFD typically is not known, but it does not appear to be inherited. Taking the drug thalidomide during pregnancy can cause CFD and other limb deficiencies in an unborn child.