The Creative Biology of Human Reproduction

Abstract The processes of procreation involve some of the oddest phenomena to be found in all of human biology. Ordinary rules are broken—cells shed half of their chromosomes, genetically distinct cells merge, and the usual boundaries between individuals are breached. This chapter introduces the basic features of male and female reproduction, including gametogenesis, implantation, and early development of the placenta and conceptus. Special attention is given to female reproductive physiology, including menarche, the menstrual cycle, and menopause. Peculiar properties of the conceptus include the ability to form identical twins, to merge non-identical twins, to implant outside the uterus, and to disperse embryonic cells into the mother’s body. The features of developmental biology provide both challenges and opportunities for epidemiologic study of early pregnancy, with implications for studies of outcomes later in pregnancy.