Implantation is associated with a major but short-lived increase in progesterone secretion at a time when circulating progesterone levels are falling in the non-fertile menstrual cycle. The working hypothesis that this luteotropic stimulus is of trophoblastic origin has been tested by administering human chorionic gonadotropin (HCG) to non-pregnant animals during the late luteal phase of the menstrual cycle. Daily intramuscular injections of HCG (17.5–500 IU) were followed by a precipitous rise in plasma progesterone concentration but this was not sustained despite continued hormone administration. The pattern of progesterone secretion observed in animals treated with 25 IU of HCG per day closely resembles that normally observed in the immediate post-implantation period. The refractoriness of the corpus luteum to continued exogenous gonadotropic stimulation is in accord with the finding that the decline in the post-implantation progesterone surge occurs in the face of a rising tide in circulating chorionic gonadotropin. The initial luteotropic stimulus of pregnancy probably is chorionic gonadotropin secreted immediately upon implantation. (Endocrinology90: 34, 1972)
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Neill et al. (1972) studied this question.