This group of malformation spans a large range of severity, from a missing phalanx of a finger or toe to complete absence (amelia) of an entire limb. At least three main categories are discerned: (a) transverse reductions ("amputations") where the end part of a limb is missing (the matching limb is usually unaffected); (b) longitudinal reductions of the outer (radial / tibial) or inner (ulnar / fibular) part of the limb -- the most common is radial aplasia, often with absence of thumb; (c) intercalary reductions are where ends of the limb are reasonably normal but the long bones of the middle part are shortened or absent (the latter form is sometimes called phocomelia, but this concept is ill-defined). Non-limb malformations may concurrently exist. Survival depends on the severity of associated malformation. Some types are part of specific syndrome, for example Holt-Oram syndrome having a cardiac defect and a radial longitudinal or an intercalary defect, sometimes associated with chromosomal anomalies. Surgical repair is usually not possible, but prosthesis may help.
There is a fairly uniform rate around the world of around 4 to 6 per 10,000 births. An increase of femoral hypoplasia, sometimes as part of the femur-fibula-ulna (FFU) syndrome has been reported from France.