Secretory (Luteal) Phase

d. Secretory phase / Luteal phase : This phase lasts from ovulation to the beginning of the next menses — it is the longest phase of the cycle, lasting about 14 days, from roughly the 15th to the 28th day. After release of the secondary oocyte, the remaining tissue of the Graafian follicle transforms, under the influence of LH, into a temporary endocrine gland called the corpus luteum, which begins secreting progesterone and estrogens. The ovulated egg may be fertilized within about 24 hours of release; but if fertilization does not occur, the corpus luteum can survive for only about two weeks before degenerating into a white scar, the corpus albicans.

The hormones released by the corpus luteum — mainly progesterone, along with smaller amounts of estrogen and inhibin — cause the endometrial glands to grow, become coiled, and begin secreting uterine fluid; the endometrium becomes increasingly vascularised and thickens to about 8–10 mm. Inhibin also suppresses further secretion of FSH. These changes together prepare the endometrium for fertilization and the subsequent implantation of the embryo.

However, if the ovulated egg does get fertilized and the resulting embryo implants, the embryo secretes human chorionic gonadotropin (hCG), which extends the life of the corpus luteum and stimulates its continued secretory activity; the presence of hCG in maternal blood and urine is, in fact, the basis of a pregnancy test. If fertilization does not occur, the corpus luteum degenerates as described, and the next menstrual cycle begins.