The menstrual cycle follows a precise pattern governed by the release of different hormones that interact with one another. Progesterone, whose production increases in the second half of the menstrual cycle, plays a pivotal part in preparing the uterine lining for implantation of an embryo. The corpus luteum, the remains of the ovulating follicle, produces progesterone after ovulation. The release of progesterone in the second half of the menstrual cycle, known as the luteal phase or secretory phase, is essential for implantation of a developing embryo.
Preparation for Pregnancy
The uterine lining, the endometrium, consists of two types of tissue, the stroma, the supporting structure and the glands, buried within the stroma, Harvey Kliman, M.D. of Yale University School of Medicine explains. During the first half of the menstrual cycle, the endometrium thickens as estrogen levels increase, which signals stroma and gland cells to divide. After ovulation, rising progesterone levels stop the proliferation of endometrial cells. Progesterone matures the stroma and glands, turning them into cells that can nourish and support a developing embryo. The blood supply to the uterus increases and the lining thickens with additional fluid and nutrients.
Preparing for New Menstrual Cycle
If an embryo doesn't implant, the corpus luteum stops producing progesterone and begins to degenerate after around 14 days, the Merck Manuals Online Medical Library explains. This causes the uterine lining to break down and slough off as the uterine contracts, which starts when progesterone levels fall. A new menstrual cycle starts again after the thickened lining sheds.
If pregnancy occurs and the embryos implants, tissue that will develop into the placenta burrows into the uterine lining and produces human chorionic gonadotropin, or hCG. Human chorionic gonadotropin, the hormone that turns urine and blood pregnancy tests positive, in turn, keeps the corpus luteum from breaking down, which keeps the production of progesterone going until the placenta takes over progesterone production later in pregnancy, retired Harvard professor John Kimball, Ph.D. states on his website, Kimball's Biology Pages. Progesterone maintains the secretory uterine lining, continuing the supply of nutrients to the developing embryo and also decreases uterine contractions that might disturb implantation.