Cases include rudimentary or seal-like limbs, incomplete or undeveloped extremities, fusion of the lower extremities, absence of the brain, abnormally small brain and head, enlargement of brain and head, protrusion of the brain through the head, absence of eyes, abnormally small eyes, defective closure of lines of junction (harelip, cleft palate, spina bifida), partial or wholly double individuals (Siamese twins), or individuals with double heads or bodies. Many such conditions may be regarded as resulting from the action of an unfavourable environment on a genetically susceptible individual. The relative importance of the environment varies with the condition.
The incidence of physical birth defects is falling in many industrialized countries due to the improvements in the resolution of prenatal ultrasound and other fetal diagnostic tools. Parents often choose to abort a child who will be born with disabilities.