Seventy-nine previously infertile women were studied during the first 9 weeks of their pregnancies. Pregnancy had occurred either without any specific therapy or following treatment with clomiphene, prednisone, or human chorionic gonadotropin, singly or in various combinations. Serum levels of progesterone, 17 alpha-hydroxyprogesterone, and estradiol were measured at 2-week intervals. A profile of early pregnancy in terms of these hormones is presented and discussed with regard to its significance for corpus luteum function. Aspects of the hormonal profile predictive of the pregnancy outcome are noted. Ovarian over-stimulation was directly related to the dosage of clomiphene, and the increased levels of 17 alpha-hydroxyprogesterone thus produced persisted into the seventh week of pregnancy. This hormonal excess probably derives from multiple corpora lutea which may also account for the increased incidence of multiple pregnancy associated with clomiphene therapy.