During ovulation, an egg is released from a dominant follicle. Following the release of the egg and subsequent fertilization, the follicle seals itself off and forms what is known as a corpus luteum. This mass of cells helps produce the hormone progesterone during early pregnancy.

The corpus luteum will continue to produce progesterone until the fetus is producing adequate levels to sustain the pregnancy, which usually occurs between 7 and 9 weeks of pregnancy.

Progesterone is essential during early pregnancy because:

• It allows the uterus to grow without having contractions
• It supports the growth of the lining of the uterus
• It improves blood flow and oxygen supply

The corpus luteum is supported and maintained by the pregnancy hormone human chorionic gonadotrophin or HCG. The corpus luteum begins to decrease in size at around 10 weeks of pregnancy.

When fertilization or implantation do not occur, the corpus luteum will begin to break down. This causes a decline in estrogen and progesterone levels, leading to the start of another menstrual period.

Corpus luteum and the menstrual cycle

Hormones play a powerful role in normal menstrual regulation in women.

As well as progesterone and estrogen, other essential hormones for ovulation include luteinizing and follicle-stimulating hormone.

These hormones are responsible for ovulation and preparing the uterus for implantation of a fertilized egg.

A typical menstrual cycle occurs every 25–36 days, at which time the body prepares for ovulation and pregnancy.

There are three phases of the menstrual cycle:

Follicular phase

The follicular phase of the menstrual cycle begins on the first day of a woman’s period, at the onset of menstrual bleeding and lasts until ovulation. This stage typically lasts anywhere from 13–14 days.

During the follicular phase, the body secretes follicle-stimulating hormone to induce the production of ovarian follicles that contain eggs. One of these follicles will grow into a mature follicle capable of being fertilized, which is known as the dominant follicle.

The dominant follicle secretes estrogen, which not only breaks down the non-dominant follicles but also stimulates the uterus to begin thickening its lining in preparation for egg implantation. It also causes the luteinizing hormone surge that is responsible for ovulation.

Ovulatory phase

The ovulatory phase begins on about day 14 of a woman’s menstrual cycle. During this time, the luteinizing hormone surges, further stimulating the ovary to release the egg from the dominant follicle.

Typically, this phase lasts anywhere from 16–32 hours, resulting in ovulation.

Luteal phase

The luteal phase of the menstrual cycle is the time where the body prepares for implantation of a fertilized egg. When an ovarian follicle releases an egg during the ovulatory phase, the opened follicle closes off, forming what is called the corpus luteum.

The corpus luteum is responsible for producing the hormone progesterone, which stimulates the uterus to thicken even more in preparation for implantation of a fertilized egg.

The hormone estrogen is also elevated during this time to prepare the uterus for implantation.

If there are no fertilized eggs to implant in the thickened uterine lining, the body sheds the lining during menstrual bleeding due to low levels of estrogen and progesterone, and the cycle begins again.

At times, the corpus luteum can fill with fluid. This buildup causes what is called a corpus luteum cyst, which is a type of functional ovarian cyst.

In most cases, corpus luteum cysts will go away on their own without treatment. Corpus luteum cysts may disappear in a few weeks or take up to three menstrual cycles to vanish altogether.

Some women who develop these cysts may experience symptoms, such as:

• Pelvic pain, which may present as dull or sharp aching
• Abdominal fullness or heaviness
• Bloating
• Aching in the lower back and thighs
• Painful intercourse
• Weight gain
• Painful menstrual bleeding
• Breast tenderness
• Need to urinate more frequently
• Difficulty emptying the bladder
• Difficulty emptying bowels completely
• Abnormal vaginal bleeding

Some women may experience a burst cyst, which can cause severe pain and possibly internal bleeding.

Larger cysts can cause the ovary to twist on itself (ovarian torsion) which can negatively affect the blood flow to the affected ovary.

At times, the corpus luteum cyst may remain past the early stages of pregnancy. If this happens, the cyst has the potential to cause problems. An obstetrician will monitor as appropriate and make referrals to specialists as necessary.

DIAGNOSIS OF CORPUS LUTEUM CYST

While some ovarian cysts can be felt during a pelvic exam, others cannot.

An obstetrician may carry out some diagnostic tests to evaluate and diagnose ovarian cysts, including:

• a pregnancy test, which may signal the presence of a corpus luteum cyst
• hormone testing
• pelvic ultrasound