The luteal phase extends from the moment of ovulation triggering until the beginning of menses, or of pregnancy. The corpus luteum is generated from luteinized granulosa and theca interna cells that remained in the follicle after oocyte extrusion. It secretes both estrogens and progesterone under the effect of a pulsatile LH stimulation that is slower (3–5 pulses/24 h) than during the follicular phase (one pulse every 90 min). If conception and nidation follow ovulation, hCG secreted by the embryonic pouch maintains the luteal gland trophicity for secretion of both steroid classes (Fig. 7.1). Between the fifth and the seventh weeks of pregnancy the placenta also begins to assume autonomous synthesis and secretion of these hormones. Csapo showed in 1973 that the pregnancy was interrupted if the corpus luteum was removed earlier than the seventh week, but progesterone administration could rescue the pregnancy by this time [1].