A hormone secreted by the ovaries that belongs to the steroid hormone family, like estrogen. Progesterone is secreted by the corpus luteum (see Corpus Luteum), the follicle that produces and releases the egg. The specialized cells of the corpus luteum, that surround the egg, produce high levels of progesterone just prior to ovulation. Progesterone also plays a major role in preparing the endometrium for implantation and supports a pregnancy through the first few weeks of gestation. However, the corpus luteum will stop secreting progesterone if pregnancy does not occur. The progesterone level in the blood will then drop, which induces menses within 24 to 48 hours. This is how a regular menstrual cycle takes place. On the other hand, if pregnancy does occur, the placenta will start secreting the HCG hormone, which will salvage the corpus luteum and cause it to continue secreting progesterone.