The complex set of changes which occur in the OVUM and in the body of the mother at the beginning of pregnancy. The precise moment of conception is that at which the male element, or spermatozoon, and the female element, or ovum, fuse together. Only one-third of these conceptions survive to birth, whilst 15 per cent are cut short by spontaneous abortion or stillbirth. The remainder – over one-half – are lost very early during pregnancy without trace. (See also FETUS.)