The corpus luteum forms from the empty follicle left behind after ovulation. It is the last active stage of an ovarian follicle’s lifecycle. The corpus luteum doesn’t get enough credit for the important role it plays in the menstrual cycle and pregnancy. Without it, early pregnancy cannot be maintained.

The ovary is made up of follicles. These tiny fluid-filled sacs each contain 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 trigger a few of these follicles to grow and the egg inside the follicles to mature.

Eventually, one follicle becomes dominant. The egg bursts from the follicle, leaving behind an empty shell of cells. Those cells go through a transformation after the egg is released, collapsing in on themselves and changing the hormones they secrete.

What Is the Corpus Luteum?

The corpus luteum is a temporary gland structure that forms after ovulation. It secretes the hormones estrogen and progesterone to prepare the body for the possibility of conception. These hormones help build up the lining of the uterus (to form a nice bed for the egg to implant into) and help maintain that lining.

If conception doesn't take place, the corpus luteum begins to break down. This leads to a drop in progesterone and estrogen, which triggers menstruation, and the reproductive cycle starts over again. When the corpus luteum is no longer active, it turns into the corpus albicans.

The Corpus Luteum and the Menstrual Cycle

There are four phases of the menstrual cycle:

• Menstruation: This phase begins with the first day of the period when the uterine lining sheds.
• Follicular phase: During the follicular phase, which overlaps with the menstrual phase, a select number of follicles in the ovary mature until one releases an egg (ovulation).
• Ovulation: Following an increase in luteinizing hormone (LH), the ovary releases the mature egg.
• Luteal phase: Post-ovulation, the body prepares the uterus to accept a fertilized egg or embryo.

Just before ovulation, LH triggers the follicle and the developing egg inside to speed up growth and development. LH also triggers enzymes to begin breaking down the outer walls of the follicle. When the egg reaches full maturity, the follicle wall breaks open releasing the mature egg.

Once the egg is released, LH continues to impact the cellular structure of the former follicle. Before ovulation, granulosa and theca cells in the follicle produce estrogen. After ovulation, LH triggers these cells to transform into the corpus luteum and they begin to release the hormone progesterone. The most important role of the corpus luteum is progesterone production to thicken uterine lining in anticipation of pregnancy implantation.

Progesterone plays an important role in the luteal phase:

• First, progesterone signals the pituitary and hypothalamus glands in the brain to slow down the production of the hormones follicle-stimulating hormone (FSH), LH, and gonadotropin-releasing hormone (GnRH), preventing additional follicles in the ovaries from developing and ovulating.
• Second, progesterone prepares the endometrium (uterine lining) by triggering the endometrium to secrete proteins. These proteins maintain the endometrium and create a nourishing environment for a fertilized egg.

The Role Corpus Luteum Plays in Conception

If an egg is fertilized and implants into the uterine lining, the embryo forms a very early placenta. This early placenta releases human chorionic gonadotropin (hCG), the hormone at-home pregnancy tests detect. The presence of hCG signals the corpus luteum to continue secreting progesterone. The progesterone prevents the endometrium from being expelled and prevents further ovulation.

If pregnancy doesn’t occur, the corpus luteum slowly disintegrates. This happens about 10 to 12 days after ovulation, or two to three days before menstruation starts. As the corpus luteum breaks down, the cells in the corpus luteum stop producing as much progesterone. Eventually, the drop in progesterone leads the endometrium to break down and menstruation begins.

The drop in progesterone also signals the pituitary and hypothalamus glands to increase the production of FSH, LH, and GnRH, which restarts the menstrual cycle and the follicular phase starts anew.

The Corpus Albicans

When the corpus luteum breaks down, it leaves scar tissue behind. This scar tissue—which is made up of cartilage—is known as the corpus albicans. While the corpus luteum is yellow in color (corpus luteum means "yellow body" in Latin), the corpus albicans is white (corpus albicans means "white body" in Latin).

The corpus albicans remains on the ovary for a few months. It doesn’t appear to serve a specific purpose. Unlike the corpus luteum, the corpus albicans doesn’t act like a gland and doesn’t secrete hormones.

Eventually, immune cells known as macrophages act like an internal clean-up crew and engulf most of the cells that once made up the corpus albicans. In very rare circumstances, the corpus albicans remains and scar tissue builds up around the ovary. Not much is understood about why this happens because it is so rare.

Corpus Luteum Cysts

The corpus luteum is formed from the open follicle that released an egg during ovulation. Sometimes, the opening seals back up and fluid fills the cavity to form a cyst. This kind of cyst is known as a functional cyst. They are usually benign (not cancerous) and go away on their own.

Corpus luteum cysts are usually painless and harmless, but if you’re going through fertility treatments and a cyst is spotted in an ultrasound, your health care provider may choose to delay your treatment cycle or drain the cyst depending on its size.

If you tend to develop corpus luteum cysts, your health care provider may recommend hormonal birth control pills for the cycle before fertility treatment. The hormones in the pill prevent ovulation before treatment, which prevents the potential for a cyst.

Some people find out they have one of these cysts during an early pregnancy ultrasound. In these cases, the cyst will usually resolve on its own by the second trimester of pregnancy. If the cyst is unusually large, growing, or painful, however, it may need to be surgically drained or removed.

Corpus Luteum Cyst Pain

While usually painless, a corpus luteum cyst can cause mild discomfort. It may come as a short, sharp twinge of pain on one side. Other times it may cause a dull, more constant pain on one side of your pelvic area. If you get pregnant, this pain may persist during the early weeks of your pregnancy. If you don’t get pregnant, the pain will likely go away a few days after your period starts.

As long as the pain is not severe and not accompanied by other worrisome symptoms (like vomiting or fever), there is probably nothing to worry about.

Corpus Luteum Deficiency or Defect

In some cases, the corpus luteum doesn’t produce enough progesterone. This can lead to abnormal spotting. Low levels of progesterone in early pregnancy may lead to what looks like a light period, making you think you’re not pregnant when you really are.

When progesterone levels are low after ovulation, this may be called a corpus luteum defect or deficiency. More commonly, it’s referred to as a luteal phase defect. A corpus luteum deficiency may increase the risk of early miscarriage.

Treatment for 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 leading up to ovulation with fertility drugs may help produce a stronger corpus luteum.

However, there's no current evidence that these treatments help and they may come with risks. For example, hCG injections increase the risk of developing ovarian hyperstimulation syndrome (OHSS). Proper diagnosis of a corpus luteum defect is also controversial and unclear. Based on the current evidence, the American Society for Reproductive Medicine doesn't recognize luteal phase defect as a specific cause of infertility.