An ovarian follicle is a fluid-filled sac in the ovary containing an egg that has not yet matured. During the menstrual cycle, an egg matures, the follicle opens,and releases the egg, preparing it for fertilization.

More than 99% of ovarian follicles disintegrate through a process called follicular atresia, preventing them from ovulating and releasing an egg. Atresia can occur at any time during follicle development.

Doctors sometimes check ovarian follicles as part of a process to assess someone’s fertility.

In this article, we examine ovarian follicles and their anatomy and development. We also look at the function of ovarian follicles in fertility and menopause and the conditions that can affect them.

A note about sex and gender

Sex and gender exist on spectrums. This article will use the terms “male,” “female,” or both to refer to sex assigned at birth. Click here to learn more.

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The ovaries are part of the female reproductive system that lie on either side of the uterus in the lower abdomen.

Ovaries have two main functions. They release an egg during ovulation, and they release estrogen and progesterone, sex hormones that support implantation and other functions.

Ovarian follicles are tiny fluid-filled sacs inside the ovaries in which eggs grow. They support ovulation by releasing an egg once it matures. If the follicle does not release a mature egg, it may swell and become a cyst.

A pelvic ultrasound scan can help medical professionals evaluate ovarian follicles. Medical professionals may also perform an antral follicle count test to check the number of follicles in an ovary and assess their size. This may serve as a fertility test to help determine a person’s likelihood of conceiving.

Folliculogenesis is the process that describes the stages of follicular development. Literature from 2015 mentions the following six stages:

  • Primordial follicle: All follicles start as primordial follicles. At birth, the ovaries contain around 1–2 million primordial follicles. It can take almost a year for primordial follicles to grow and develop to the ovulatory stage. Primordial follicles are surrounded by cells called granulosa cells, which are the primary producers of estrogen.
  • Primary follicle: Some primordial follicles move to the primary stage every day. This process starts during puberty and lasts until menopause.
  • Secondary follicle: Granulosa cells start to increase estrogen production. The inside of the follicle also forms tiny pockets of nutrient-rich follicular fluid. At a later stage, this forms the antrum, which is the follicle’s fluid-filled cavity.
  • Tertiary follicle: Another name for tertiary follicles is antral follicles. Their development occurs through the secondary follicles that form the antrum. Doctors can view the tertiary follicles on an ultrasound scan at this stage.
  • Graafian follicle: This is the stage in which the mature follicle can now ovulate.
  • Corpus luteum: Once ovulation occurs and the follicle is empty, it transforms into the corpus luteum. The corpus luteum consists of follicular theca cells and granulosa cells. It is responsible for producing progesterone during early pregnancy.

When females reach puberty, they have about 400,000–500,000 primordial follicles. After the first menstrual cycle, females lose around 1,000 follicles every month, increasing after age 35.

It is impossible to determine the number of eggs in an ovary. Primordial follicles are microscopic, so medical professionals cannot see them on an ultrasound scan. However, they become visible once they reach the tertiary stage.

Usually, only one follicle matures into an egg, occurring around the 10th–18th day of the menstrual cycle.

Females have approximately 2,000,000 eggs in their ovaries when they are born. As they reach puberty, the number decreases to around 400,000. At menopause, ovaries have about 1,000 remaining eggs.

There is only one egg in each follicle, and the number of follicles makes up a person’s ovarian reserve.

However, the authors of a 2018 study explain that not all follicles reach the mature phase to release an egg.

A decrease in mature follicles leads to low ovarian reserve status, meaning that there is a low chance that a person will conceive.

Physicians use a transvaginal ultrasound scan to determine the follicle count.

Menopause is when females no longer have periods and can no longer become pregnant naturally.

Those who have gone through menopause may experience vaginal dryness. Hormonal levels may also be low, causing the body to produce less estrogen and progesterone.

The number of ovarian follicles reduces as individuals go through menopause, and the follicle loss rate increases during the last 10 years of the menstrual cycle.

Some of the following conditions can affect ovarian follicles:

Polycystic ovarian syndrome (PCOS)

PCOS affects 6–12% of females of reproductive age in the United States. The Centers for Disease Control and Prevention (CDC) state that PCOS can cause a release of androgens (male sex hormones that females also have), acne, and excessive body hair growth.

People with this condition have an excess of immature ovarian follicles in their ovaries. PCOS prevents follicle growth, so they cannot release egg cells.

Learn more about PCOS here.

Functional cysts

Functional cysts can develop if the follicle does not release an egg or fluid. These are not cancerous, but they may cause pelvic pain.

Conditions that could lead to the formation of ovarian cysts include PCOS and endometriosis.

Some people may not require treatment, as the cysts tend to disappear without intervention.

Learn more about ovarian cysts here.

Primary ovarian insufficiency

Primary ovarian insufficiency or premature ovarian failure occurs when the ovaries stop working before age 40.

This condition causes irregular periods and reduced fertility. It may start for some people during adolescence.

Premature ovarian failure may occur in those with a low number of follicles or genetic disorders.

Learn more about primary ovarian insufficiency here.

Ruptured ovarian cysts

Most ovarian cysts are harmless and are common in individuals of reproductive age.

People with a ruptured cyst may develop:

Some people may require surgery for complications that arise from a ruptured ovarian cyst.

Learn more about ruptured ovarian cysts here.

Medical professionals may use the following tests to diagnose conditions that affect ovarian follicles:

  • Pelvic ultrasound: This scan helps doctors check the size of the ovaries and the number of follicles a person has.
  • Blood tests: These can help measure hormone levels and check if an individual has diabetes or high cholesterol levels. Hormone level tests can help determine the condition causing high hormone production.
  • Laparoscopy: This is a surgical procedure that a person has under anesthesia. A doctor inserts a specialist instrument into the abdomen to examine the cysts and ovaries.

Doctors may recommend the following different treatment options, depending on the condition an individual develops:

  • Supplements: Calcium and vitamin D supplements may benefit people with primary ovarian insufficiency. This is because they may have a high risk of developing osteoporosis.
  • Birth control pill: The birth control pill can help regulate menstrual periods, treat PCOS, and reduce the risk of ovarian cancer.
  • Lifestyle changes: The CDC recommends that people have testing for type 2 diabetes if they have PCOS. A doctor may suggest exercising regularly and aiming to reach or maintain a moderate weight.

Ovarian follicles support egg growth. Doctors sometimes check them when people have difficulty conceiving.

Some conditions that can affect ovarian follicles include PCOS and cyst rupture.

A person should talk with a doctor if they develop symptoms such as irregular bleeding, pelvic or, back pain, or wish to know more about their fertility.