The corpus luteum is a temporary gland that forms on the ovary after ovulation. Its existence and subsequent fate—either rapid degeneration or prolonged survival—provide the two distinct answers to when it breaks down. This structure plays a central role in reproductive health, acting as an endocrine factory that prepares the body for a potential pregnancy within each menstrual cycle.
Function and Purpose of the Corpus Luteum
The corpus luteum develops from the remnants of the ovarian follicle immediately after a mature egg is released during ovulation. This transformation is driven by the surge of Luteinizing Hormone (LH), which causes the remaining follicular cells to reorganize and enlarge, a process called luteinization. The resulting structure is typically yellow in color, which is where its Latin name, meaning “yellow body,” originates.
The primary function of this temporary gland is the production and secretion of the steroid hormone progesterone. Progesterone is responsible for initiating the luteal phase of the menstrual cycle, the approximately 14-day period following ovulation. This hormone acts on the uterus, causing the inner lining, the endometrium, to become thicker, highly vascularized, and rich in nutrients, creating an optimal environment for embryo implantation.
Progesterone also helps to inhibit the release of more eggs by suppressing the production of Follicle-Stimulating Hormone (FSH) and LH from the pituitary gland. This hormonal action ensures that the body focuses on maintaining the current potential pregnancy rather than beginning a new cycle. The corpus luteum also produces moderate levels of estradiol and inhibin A, which contribute to the overall hormonal balance necessary for reproductive function.
Degeneration in the Absence of Pregnancy
When fertilization does not occur, or when a fertilized egg fails to implant, the corpus luteum begins to degenerate, a process known as luteolysis. This is the most common scenario, occurring approximately 10 to 14 days after ovulation, typically starting around days 22 to 24 of a 28-day cycle.
The trigger for this breakdown is the natural decline in Luteinizing Hormone (LH) support, combined with the lack of a rescue signal from an embryo. As the LH levels fall, the luteal cells lose their ability to produce progesterone efficiently.
The process of structural luteolysis is initiated by local luteolytic factors, including Prostaglandin F2 alpha (PGF2a). This prostaglandin reduces blood flow to the corpus luteum and triggers programmed cell death, or apoptosis, within the luteal cells. The reduction in blood supply and subsequent cellular death cause the structure to rapidly shrink and lose hormonal activity.
Preservation During Early Pregnancy
The corpus luteum’s degeneration is prevented if successful fertilization and implantation occur. Preservation begins around the time of implantation, approximately seven to nine days after ovulation, when the newly formed embryonic tissue begins to secrete a hormone that acts as a rescue signal.
This signal is Human Chorionic Gonadotropin (hCG), the hormone detected by home pregnancy tests. hCG is structurally similar to LH and binds to the LH receptors on the corpus luteum cells, effectively taking over the role of LH and stimulating the corpus luteum to continue growing and increase its production of progesterone.
When the corpus luteum is preserved by hCG, it is sometimes referred to as the corpus luteum of pregnancy, or corpus luteum graviditatis. This preservation is sustained until the developing placenta is mature enough to take over the role of progesterone production, a transition known as the luteoplacental shift.
This shift typically occurs between the 8th and 12th week of gestation. Once the placenta is fully functional, the corpus luteum’s role diminishes, and it begins its slow, natural degeneration.
The Outcome of Luteolysis
Whether degeneration is triggered by the end of a non-pregnant cycle or the handoff to the placenta, the functional degeneration—the sharp drop in progesterone and estradiol levels—is the immediate hormonal outcome of luteolysis. This decline means progesterone can no longer support the thick, nutrient-rich uterine lining.
The breakdown of the endometrium, which the corpus luteum worked to establish, leads directly to menstruation, signaling the start of a new menstrual cycle. The drop in progesterone removes the hormonal suppression on the pituitary gland, allowing levels of FSH and LH to rise again. This rise initiates the follicular phase, stimulating the growth of a new cohort of ovarian follicles for the next potential ovulation.
Structurally, the remnants of the degenerating corpus luteum are replaced by a mass of white, fibrous scar tissue called the corpus albicans. The corpus albicans is a permanent, non-functional scar on the surface of the ovary, marking the site of a past ovulation event.