The corpus luteum is a temporary hormone-producing structure that forms on the ovary after ovulation. Its primary job is to release progesterone, the hormone that prepares the uterine lining for a potential pregnancy. If fertilization occurs, it sustains the early pregnancy until the placenta takes over. If it doesn’t, the corpus luteum breaks down, progesterone drops, and menstruation begins.
How the Corpus Luteum Forms
Every month during ovulation, a mature follicle on the ovary ruptures and releases an egg. What’s left behind doesn’t go to waste. The remnant follicle collapses, and the cells that once surrounded the egg undergo a rapid transformation triggered by a surge of luteinizing hormone (LH) from the brain. Two types of follicular cells, called granulosa and theca cells, reorganize into a new structure: the corpus luteum. Blood vessels from the surrounding tissue invade the collapsed follicle, giving the corpus luteum a rich blood supply that supports its intense hormone production.
Before ovulation, these two cell types had distinct, complementary roles in producing estrogen. After the LH surge, both cell types shift their machinery to prioritize progesterone production instead. The transformed granulosa cells become large luteal cells, and the theca cells become small luteal cells. Together, they turn the corpus luteum into a small but powerful endocrine gland sitting right on the surface of the ovary.
Progesterone and Its Effects on the Uterus
Progesterone is the corpus luteum’s signature product, and its target is the endometrium, the lining of the uterus. During the first half of the menstrual cycle, estrogen thickens the endometrium and promotes cell growth. Progesterone then takes over in the second half, transforming that estrogen-primed lining into a nutrient-rich environment capable of supporting an embryo. It increases blood supply to the uterine wall, stimulates glandular secretions, and stabilizes the tissue so it’s ready for implantation.
Progesterone also actively dials down estrogen’s influence on the endometrium. It reduces the number of estrogen receptors in uterine tissue and ramps up enzymes that convert active estrogen into weaker forms. This shift is essential: without it, the lining would continue proliferating rather than maturing into a state that can accept and nourish a fertilized egg. The corpus luteum also produces estrogen, but progesterone is the dominant hormone during this phase.
What Happens Without Pregnancy
If the egg isn’t fertilized, the corpus luteum has a built-in expiration date. About eight days after ovulation, it begins breaking down in a process called luteolysis. This is triggered by prostaglandin F2-alpha released from the uterus, which causes the luteal cells to shrink and die off. Immune cells, including macrophages, move in to clean up the debris, while fibroblasts lay down dense connective tissue in its place.
As the corpus luteum degrades, progesterone and estrogen levels fall sharply. Without progesterone to maintain it, the thickened uterine lining breaks down and sheds, producing a menstrual period. This drop in hormone levels also releases the brain from negative feedback, allowing a new cycle of follicle development to begin. The remnant left behind on the ovary is a small scar of dense connective tissue called the corpus albicans. These scars accumulate over a lifetime. Under a microscope, a postmenopausal ovary is dotted with them, each one a marker of a past ovulation.
Its Role in Early Pregnancy
When a fertilized egg implants in the uterine lining, the developing embryo sends a rescue signal: human chorionic gonadotropin, or hCG. This is the same hormone detected by pregnancy tests. hCG binds to the same receptors as LH on the corpus luteum, keeping it alive and actively producing progesterone. Without this signal, the corpus luteum would degrade on schedule, progesterone would drop, and the pregnancy would be lost.
The corpus luteum remains the primary source of progesterone through roughly the first trimester. During this window, the placenta is still developing and can’t yet produce enough hormones on its own. In a typical singleton pregnancy, the transition from corpus luteum to placenta as the main progesterone source happens between about 6 and 8 weeks of gestation, when hCG peaks and then declines. In twin pregnancies, this shift takes longer, with hCG reaching higher concentrations and not peaking until 10 to 12 weeks. Once the placenta fully takes over hormone production, the corpus luteum is no longer needed and gradually regresses.
Corpus Luteum Cysts
Sometimes the corpus luteum fills with fluid or blood and forms a cyst. These are common and usually harmless, typically measuring between 1 and 3 centimeters. They develop when a small blood vessel on the ovary’s surface is disrupted during ovulation, causing blood to pool inside the structure. A blood-filled corpus luteum cyst can cause pelvic pain or a dull ache on one side and generally takes longer to resolve than a fluid-filled one. Most disappear on their own within a few weeks without treatment.
When the Corpus Luteum Underperforms
A condition called luteal phase deficiency occurs when the corpus luteum doesn’t produce enough progesterone to properly prepare the uterine lining. This can make implantation difficult or increase the risk of early miscarriage. It’s a relatively uncommon but clinically significant cause of infertility. Diagnosis typically involves measuring progesterone levels in the middle of the luteal phase, roughly a week after ovulation. A single reading below 10 ng/mL, or a sum of three separate readings totaling less than 30 ng/mL, suggests insufficient progesterone output. Treatment usually focuses on supplementing progesterone to compensate for what the corpus luteum isn’t providing.