The corpus luteum is a temporary endocrine structure that forms in an ovary after ovulation from the cells of the follicle that released an egg. Its name is Latin for “yellow body,” referring to its yellowish color from concentrated dietary carotenoids. This structure is a normal part of the menstrual cycle during a person’s reproductive years.
The Lifecycle of a Corpus Luteum
The journey of the corpus luteum begins in the second half of the menstrual cycle, known as the luteal phase, immediately following ovulation. Once the dominant follicle in the ovary ruptures to release an egg, the remaining follicular cells undergo a rapid transformation. This process, called luteinization, reshapes these cells into a new, functional gland that can grow to be between 2 to 5 centimeters in diameter.
From this point, the corpus luteum faces one of two distinct fates, determined by whether fertilization occurs. If the egg is not fertilized, the corpus luteum has a limited lifespan of about 10 to 14 days. Without a signal from a developing embryo, it begins to break down in a process called luteolysis. This degeneration causes a sharp decline in the hormones it was producing, which triggers the shedding of the uterine lining and the start of menstruation.
Following its breakdown, the functional tissue of the corpus luteum is replaced by fibrous scar tissue. This remnant is known as the corpus albicans, or “white body,” a non-functional scar that remains on the surface of the ovary.
Primary Hormonal Functions
The main purpose of the corpus luteum is to produce hormones that prepare the body for a potential pregnancy. Its most significant product is progesterone, a steroid hormone that acts directly on the uterus. Progesterone causes the uterine lining, or endometrium, to thicken, become more vascular, and develop glands that secrete nutrients.
Progesterone also has a broader influence on the menstrual cycle. It sends signals back to the brain that inhibit the release of hormones that would otherwise stimulate the development of new ovarian follicles. This action prevents further ovulation from occurring within the same cycle.
While progesterone is the primary hormone, the corpus luteum also secretes smaller amounts of estrogen and inhibin A. Estrogen assists progesterone in preparing the uterine lining. Inhibin A also contributes to the suppression of follicle-stimulating hormone from the pituitary gland, reinforcing the block on new ovulations.
Role in Early Pregnancy
When a fertilized egg implants in the uterine wall, the embryo produces human chorionic gonadotropin (hCG), the hormone detected in pregnancy tests. hCG signals the corpus luteum to continue functioning, rescuing it from the degeneration it would otherwise undergo.
In response to hCG, the corpus luteum continues producing progesterone. These sustained high levels of progesterone are needed during the first trimester to maintain the uterine lining, improve blood flow to the fetus, and suppress uterine contractions.
The corpus luteum remains the primary source of progesterone for approximately the first 7 to 9 weeks of gestation. After this period, the placenta has developed sufficiently to take over the production of progesterone. This transition is often called the luteal-placental shift. Once the placenta is in control, the corpus luteum is no longer needed, and it slowly degrades into a corpus albicans around the 12th week of pregnancy.
Associated Medical Conditions
Occasionally, issues can arise with the corpus luteum. The most common condition is the formation of a corpus luteum cyst, which occurs when the structure fills with fluid and does not break down as expected. These cysts are a type of functional ovarian cyst and are usually harmless and painless, often resolving on their own. However, larger cysts can sometimes cause symptoms like pelvic pain, bloating, or aching in the lower back.
Another, less common issue is known as a luteal phase defect (LPD). This condition occurs when the corpus luteum does not produce an adequate amount of progesterone, or when the uterine lining does not respond properly to it. This progesterone insufficiency can prevent the endometrium from supporting an embryo, which may affect fertility.