Progesterone (P) is an essential hormone to allow the establishment of a pregnancy and to prevent spontaneous abortion during the first trimester. One way to treat P deficiency is simply to supplement extra P during the luteal phase and during the first trime­ster. Another way to increase serum P is to use follicle maturing drugs. However, these latter drugs may also cause an increase in contra-P hormones so that luteal phase deficiency is common when anovulatory women are treated with follicle maturing drugs Improved pregnancy and especially reduced miscarriage rates, will be found when P is supplemented in the luteal phase in women requiring follicle maturing drugs. When women have fairly regular cycles and appear to be ovulating they may still have endometrial biopsies that are out of phase and be classified as having luteal phase defects (LPD). A slight majority of women with LPD make mature follicles and respond much better to supplemental P than to follicle maturing drugs. In some cases the use of follicle maturing drugs creates a hostile uterine environment possibly related to premature trophoblast invasion.