What Is the Corpus Luteum?

Medically Reviewed by Mahammad Juber, MD on October 14, 2022
5 min read

Your reproductive system is designed to prepare for pregnancy every month during your childbearing years. This is a collaborative effort of your reproductive organs and relevant hormones. The corpus luteum is a vital yet transient organ that plays a key role in this process by producing an essential hormone, progesterone.

Read on to learn everything you need to know about the corpus luteum location, anatomy, functions, and more.

Your internal reproductive organs include two ovaries, two fallopian tubes, one uterus, and a vagina. The corpus luteum may be located in either of the ovaries, depending on which one is participating in the current menstrual cycle. 

The corpus luteum is a small, saffron-yellow cyst-like structure, approximately 2 to 5 centimeters in size. The corpus luteum comprises a mass of cells of two types — follicular granulosa cells and follicular theca cells. During pregnancy, these cells maintain progesterone secretion under the influence of the hormone human chorionic gonadotropin, which is produced by the developing embryo.

To better understand the purpose and development of the corpus luteum, let’s discuss the menstrual cycle. It usually lasts for a month and has three main phases:

  1. Follicular phase. In this 14-day phase, the follicle-stimulating hormone (FSH) released by your pituitary gland urges small sacs called follicles in your ovaries to produce an egg or ovum. Usually, only one ovary produces an ovum during each cycle. Many follicles in an ovary respond to this stimulation, but the most responsive follicle is called the dominant follicle. An egg forms within it, whereas the remaining follicles shrink and disappear. 
  2. Ovulatory phase. In this phase, ovulation takes place over one to two days. After the egg is developed in the dominant follicle, the pituitary gland secretes luteinizing hormone (LH), which is a signal to the ovarian follicle to release the ovum into the fallopian tube. This tube is connected to the upper part of the uterus. Once sperms enter the vagina, they travel through the uterus to reach the fallopian tube. If this happens during the ovulatory phase, they have a chance to meet the egg and fertilize it.
  3. Luteal phase. In this 14-day phase, once the egg has left the ovary, the remnant follicle cells form the corpus luteum. Depending on whether fertilization has occurred, the corpus luteum exhibits different behaviors and is eventually discarded at the end of the cycle. This means that over the course of your reproductive years, you might produce hundreds of corpus luteums.

Once your corpus luteum is developed, it can produce the hormones estrogen and progesterone during the luteal phase. Progesterone is critical in this phase because it makes your uterine environment suitable for pregnancy in the following ways:

  • Thickens the inner lining of the uterus (endometrium) so that the fertilized egg can be safely implanted
  • Modifies the pattern of proteins produced by the endometrial lining cells to support fetal growth
  • Improves the blood flow and oxygen supply to the uterus so that a fetus can develop well
  • Increases the size of the uterus 
  • Relaxes the uterine muscles to prevent contractions that could expel the fertilized egg or embryo early

If the egg is not fertilized within approximately 10 days after ovulation, the corpus luteum begins to degenerate. Without the support of progesterone to maintain it, the uterus sheds its inner lining, resulting in menstrual bleeding (or a period). This is also called the menstrual phase.

If the egg is fertilized, the corpus luteum remains intact and continues to release progesterone for the first 12 weeks of pregnancy. In these 12 weeks, the placenta develops along with the fetus in the uterus. Once created, it takes over the corpus luteum function of hormone production to support the pregnancy. The corpus luteum then shrinks and eventually breaks down.

In addition to progesterone, the corpus luteum produces the hormone relaxin during pregnancy. This helps to soften your pelvic joints, which is crucial for smooth vaginal delivery.

In the event of pregnancy, the corpus luteum also sends signals to your brain to stop pituitary hormone production so that the menstrual cycle can halt for the duration of the pregnancy.

Now that we’ve discussed the usual functions of the corpus luteum, let’s see what happens when things don’t go as planned. 

If the corpus luteum can’t produce progesterone as expected, this is called a corpus luteum defect. If the corpus luteum produces progesterone, but the uterus can’t respond to it, this is called a luteal phase defect. 

These may occur due to stress, malnutrition, obesity, overexercise, or other medical conditions like thyroid disease, polycystic ovary syndrome (PCOS), and endometriosis. Without progesterone to support the early stages of pregnancy, you may find it difficult to conceive, resulting in infertility or repeated pregnancy loss.

If your physician suspects luteal phase deficiencies, they’ll check your hormone levels (mainly FSH, LH, and progesterone) and perform an ultrasound examination to assess the thickness of your uterine lining.

Occasionally, the corpus luteum doesn’t break down on time and becomes filled with fluid. This is called a corpus luteum cyst. They’re usually benign and asymptomatic. They can develop at any time but are often noticed during a routine pregnancy exam. 

If you’re pregnant, these cysts resolve by the second trimester. If you’re not, they usually disappear within a few weeks to three months. These cysts can continue producing progesterone. Sometimes they could burst, resulting in internal bleeding. Seek medical assistance in such cases. 

Other symptoms that you might rarely experience include pain during intercourse, defecating, or urinating, pain in your low back, hips, or one side of the body, a feeling of fullness in the abdomen, bloody vaginal discharge, and breast tenderness.

These conditions are easily treatable. Doctors may recommend cyst removal if it's too big or painful. For corpus luteum defects, your physician may suggest hormone therapy, progesterone supplements, or medications like clomiphene citrate based on your situation.

The corpus luteum may be tiny, but it has a powerful role in getting you pregnant and maintaining your pregnancy. Educate yourself on your menstrual cycle so you can pick up on any changes and seek medical advice early on.

Keeping your corpus luteum healthy is simple and boils down to this — eat well, sleep well, manage your stress, and exercise regularly.