Progesterone, the naturally-occurring 21-carbon steroid produced by the corpus luteum of the ovary and by the placenta, plays an important role in re-production. It is not surprising, then, that it has been used extensively as a supplement in the treatment of disorders of fertility and pregnancy. For example, progesterone is necessary for maintenance of pregnancy in mammals, including humans, and has been given for threatened or habitual miscarriage. The relaxant effect of progesterone on smooth muscle, including that of the uterus, has led to administration of this agent for preterm labor. The effect of progesterone in preparing the endometrium for implantation of the blastocyst has given rise to use of this hormone after natural or induced ovulation to supplement the function of the corpus luteum. How well such treatments work has been argued for many years; however, the fact remains that large numbers of women of reproductive age have received this hormone. It is pertinent, then, to consider whether progesterone or similar agents exert adverse effects on embryo development.