Progestin is secreted at the level of the ovary, primarily by luteinized follicles. Progestin levels increase just prior to ovulation and peak five to seven days post-ovulation.
Progestins stimulate the release ofenzymes from cells that ultimately prepare for ovulation. Progestins further make migration of blood vessels into the follicle wall and stimulate secretion in follicular tissues. During the luteal phase, progestins causes swelling and increased secretion of the endometrium (uterrine lining).