The corpus luteum is formed from the empty follicle remaining after ovulation. This is the last active phase of the ovarian follicle life cycle. The corpus luteum is not sufficiently appreciated for the important role it plays in the menstrual cycle and pregnancy. Without it, early pregnancy cannot be sustained.
The ovary is made up of follicles. Each of these small, fluid-filled sacs contains an immature egg (or oocyte). During the first two phases of the menstrual cycle, hormones regulated by the hypothalamus and released by the pituitary gland cause a small number of these follicles to grow and the eggs within the follicles to mature.
Eventually, one follicle becomes dominant. The egg bursts from the follicle, leaving behind a shell devoid of cells. After the egg is released, these cells change shape and flip over, changing the hormones they secrete.
What is the corpus luteum?
The corpus luteum is a temporary glandular structure that forms after ovulation. It secretes the hormones estrogen and progesterone to prepare the body for the possibility of pregnancy. These hormones help build the lining of the uterus (creating a good substrate for the implantation of an egg) and help maintain this lining.
If fertilization does not occur, the corpus luteum begins to break down. This leads to a drop in progesterone and estrogen, which causes menstruation to begin and the reproductive cycle to begin again. When the corpus luteum is no longer active, it transforms into the corpus albicans.
The corpus luteum and the menstrual cycle
The menstrual cycle has four phases:
- Menstruation : This phase begins on the first day of menstruation, when the lining of the uterus is shed.
- Follicular phase : During the follicular phase, which overlaps with the menstrual phase, a number of follicles mature in the ovary until an egg is released (ovulation).
- Ovulation : After a surge in luteinizing hormone (LH), the ovary releases a mature egg.
- Luteal phase : After ovulation, the body prepares the uterus to receive a fertilized egg or embryo.
Just before ovulation, LH stimulates the follicle and the developing egg inside to accelerate growth and development. LH also stimulates enzymes to break down the outer walls of the follicle. When the egg reaches full maturity, the follicle wall opens and the mature egg is released.
When the egg is released, LH continues to influence the cellular structure of the previous follicle. Before ovulation, the granulosa and theca cells in the follicle produce estrogen. After ovulation, LH stimulates these cells to develop into the corpus luteum, which begins to secrete the hormone progesterone. The most important role of the corpus luteum is to produce progesterone to thicken the uterine lining in anticipation of implantation of a pregnancy.
Progesterone plays an important role in the luteal phase:
- First, progesterone signals the pituitary gland and hypothalamus in the brain to reduce the production of follicle-stimulating hormone (FSH), LH, and gonadotropin-releasing hormone (GnRH), thereby preventing the growth and ovulation of extra follicles in the ovaries.
- Second, progesterone prepares the endometrium (lining of the uterus) by stimulating the endometrium to secrete proteins. These proteins maintain the endometrium and create a nourishing environment for a fertilized egg.
The role of the corpus luteum in conception
If the egg is fertilized and implants in the uterine wall, the embryo forms a primary placenta. This early placenta releases human chorionic gonadotropin (hCG), a hormone that home pregnancy tests detect. The presence of hCG signals the corpus luteum to continue secreting progesterone. Progesterone prevents the endometrium from shedding and prevents further ovulation.
If pregnancy does not occur, the corpus luteum slowly breaks down. This occurs about 10 to 12 days after ovulation or two to three days before the start of menstruation. When the corpus luteum breaks down, the corpus luteum cells stop producing progesterone. The decline in progesterone eventually leads to the breakdown of the endometrium and the onset of menstruation.
The drop in progesterone also signals the pituitary gland and hypothalamus to increase production of FSH, LH, and GnRH, which restarts the menstrual cycle and the follicular phase begins again.
Corpus Albicans
When the corpus luteum breaks down, a scar is left behind. This scar tissue—which is made up of cartilage—is known as the corpus albicans. While the corpus luteum is yellow (corpus luteum means “yellow body” in Latin), the corpus luteum is white (corpus albicans means “white body” in Latin).
The corpus albicans remains on the ovary for several months. It does not appear to serve any particular purpose. Unlike the corpus luteum, the corpus albicans does not function as a gland and does not secrete hormones.
Finally, immune cells called macrophages act as an internal cleaning crew that engulfs most of the cells that once made up the corpus albicans. In very rare cases, the corpus albicans remains and a scar forms around the ovary. It’s not well understood why this happens, because it’s so rare.
Corpus luteum cyst
The corpus luteum is formed from an open follicle that releases an egg during ovulation. Sometimes the opening opens and fluid fills the cavity, forming a cyst. This type of cyst is known as a functional cyst. They are usually benign (noncancerous) and go away on their own.
Corpus luteum cysts are usually painless and harmless, but if you are undergoing infertility treatment and a cyst is seen on an ultrasound, your doctor may delay or abort your treatment cycle depending on the size of the cyst.
If you are prone to developing corpus luteum cysts, your doctor may recommend hormonal birth control pills for the cycle before fertility treatment. The hormones in the pill prevent ovulation before treatment, which prevents the possibility of cysts forming.
Some people find out they have one of these cysts during an early pregnancy ultrasound. In these cases, the cyst usually resolves on its own by the second trimester of pregnancy. If the cyst is unusually large, growing, or painful, it may need to be drained or removed.
Corpus luteum cyst pain
While a corpus luteum cyst is usually painless, it can cause mild discomfort. It may appear as a brief, sharp pain on one side. Other times, it can cause a more vague, constant pain on one side of your pelvic area. If you become pregnant, this pain may persist into the early weeks of pregnancy. If you don’t become pregnant, the pain will likely go away a few days after your period starts.
As long as the pain is not severe and is not accompanied by other worrisome symptoms (such as vomiting or fever), there is probably nothing to worry about.
When to see a healthcare provider
In rare cases, a corpus luteum cyst can cause severe pain. In very rare cases, if the cyst becomes particularly large, it can cause the ovary to twist. This can lead to ovarian torsion, a twisting that restricts blood flow to the ovary, requiring surgery. If you experience severe pain or unusual bleeding, call your healthcare provider immediately or go to the nearest emergency room.
Lack or defect of the corpus luteum
In some cases, the corpus luteum does not produce enough progesterone. This can lead to abnormal spotting. Low progesterone levels in early pregnancy can lead to light periods and make you think you are not pregnant when in fact you are.
When progesterone levels are low after ovulation, it may be a case of corpus luteum deficiency. This is called a luteal phase defect. Corpus luteum deficiency can increase the risk of premature miscarriage.
Treatment for a luteal phase defect may include progesterone supplementation or the use of fertility drugs such as Clomid (clomiphene) or hCG injections. The theory behind these treatments is that boosting the hormones that lead to ovulation with fertility drugs can help create a stronger corpus luteum.
However, there is no current evidence that these treatments help and they may carry risks. For example, hCG injections increase the risk of ovarian hyperstimulation syndrome (OHSS). The correct diagnosis of corpus luteum deficiency is also controversial and uncertain. Based on current evidence, the American Society for Reproductive Medicine does not recognize luteal phase defect as a specific cause of infertility.
