The foetal period lasts from completion of membrane attachment to birth. Development of most organs, body parts and extra embryonic membranes occurs in the embryo, which develops from the zygote, which is weakly, attached to the uterine wall. Embryogenesis involves cytodifferentiation and complex patterns of cell proliferation and migration to form organs and tissues. Developmental malformation is known as teratogenesis and may be a result of genetic defects or caused by teratogenic agents.
Some diseases are congenital and are therefore present at birth.
Hereditary diseases are also present at birth although they may not be detectable until later. The instructions for the expression of the disease are acquired from the DNA of the parents and are represented in the genome of the offspring. Lethal genes are recessive which in the homozygous state cause death of the zygote, embryo or foetus or prevent survival after birth. The lesion may be evident grossly as a malformation or may only be demonstrated biochemically. In the absence of infection, and with a closed cervix, the foetus may become progressively dehydrated .