How Ovarian and Antral Follicles Relate to Fertility

In the?ovaries of the?female reproductive system, an ovarian follicle is a fluid-filled sac that contains an immature egg, or oocyte.?During ovulation, a mature egg is released from a follicle. While several follicles begin to develop each cycle, normally only one will ovulate an egg (rarely, more than one egg is released, increasing the potential for twins).?After ovulation, the follicle?turns into a?corpus luteum.

The follicles that do not release a mature egg disintegrate, a process known as atresia that?can happen at any stage of follicular development. About 99% of ovarian follicles will disintegrate and never become mature enough to release an egg.??

Follicle growth and development are tracked during fertility treatments. During superovulation (used during IVF treatment), the goal is to stimulate the ovaries to develop several mature follicles at once. An?ultrasound exam, also known as an antral follicle count (AFC), may be performed as a part of?fertility testing. This test is done to evaluate ovarian reserves.

Antral follicle count test
Verywell / Gary Ferster?

What Are Antral Follicles?

Theoretically, if you could know how many follicles are inside your ovaries, you could have an idea of how many eggs you have left. It is impossible to count how many follicles are in the ovaries because they are too small to be visualized. However, once a follicle reaches a certain stage, it can be seen via ultrasound.

Follicles start off very, very small. All the follicles in the ovary start off as primordial follicles. A primordial follicle is just 25?micrometers—that's 0.025 millimeters. It is impossible to see with the naked eye, let alone on an ultrasound. Every day, primordial follicles are “awakened” by hormonal signals and?start to mature. As long as they continue to survive and graduate to the next stage, they grow larger and larger.

One of those stages is the tertiary stage. During this time, the follicle gains a fluid-filled cavity known as the antrum. Follicles with an antrum are referred to as antral follicles and?measure between 2 and 10 mm in diameter. For some perspective, an antrum follicle that is now 5 mm is?200 times bigger?than it was as a primordial follicle.

Antrum follicles are visible with ultrasound. Research has found that the number of active antrum follicles on the ovaries correlates to the potential number of eggs left.?? You still can’t know how many follicles total there are, but when very few antral follicles are developing on the ovaries, ovarian reserves are likely to be low.

Antral follicles produce higher levels of a hormone known as anti-mullerian hormone?(AMH), which circulates in the blood. Measuring AMH levels via blood testing is another way to evaluate ovarian reserves. Unlike FSH and estradiol (most accurate on day 2 of the cycle), AMH levels can be drawn at any time of the cycle).

The Antral Follicle Count (AFC) Test

An antral follicle count is done via transvaginal ultrasound, sometimes between cycle day 2 and 5. The test may be done as part of a fertility workup. Or, it may be ordered before a fertility treatment cycle.

During this test, the ultrasound technician looks at each ovary and counts the number of follicles measuring between 2 and 10 mm.?? Your doctor may order this test to:

  • Evaluate ovarian reserves
  • Make a diagnosis of primary ovarian insufficiency
  • Help diagnose?polycystic ovarian syndrome
  • Get an idea of where your fertility stands in relation to your age (whether you have more, less, or about the same number of follicles compared to others your age)

How Many Antral Follicles Are Normal?

It is normal for your ovarian reserves to decrease as you age. So what’s normal for a?25-year-old?isn’t necessarily normal for a?38-year-old. Still, an antral follicle count of 3 to 6 is considered low.

One?classic study conducted antral follicle counts in women with proven fertility (most studies on AFC were done on infertile women).?? To be included in this study, the women had to:

  • Have?regular menstrual cycles, varying from 21 to 35 days in length
  • Be ovulating, proven by having a temperature rise on a?basal body temperature chart
  • Have conceived at least one child in the past, within one year’s time
  • Have no known hormonal or endocrinological disease
  • Stopped birth control?at least two months before the study

Here were their average antral follicle counts (AFC), along with the range seen (from the lowest to highest AFC count).

Age range Average AFC Lowest AFC Highest AFC
25 to 34 15 3 30
35 to 40 9 1 25
41 to 46 4 1 17

Having low ovarian reserves doesn’t mean you can’t get pregnant. But it does mean your ovaries may not respond to fertility drugs as well as a woman with better ovarian reserves. The skill of the ultrasound technician and the ultrasound equipment itself can affect the results. If one test shows a poor result, consider getting a second opinion.

Women with a very low antral follicle count before age 40 may be diagnosed with primary ovarian insufficiency, also known as premature ovarian failure.?? Antral follicle counts naturally lower as a woman ages. An unusually high antral follicle count may indicate polycystic ovarian syndrome (PCOS).

The Role Follicles Play in the Menstrual Cycle

Your menstrual cycle is split into two primary parts: the follicular phase and the luteal phase. During the follicular stage, follicles in the tertiary stage of development are recruited and begin a process that will eventually lead to ovulation.

While several follicles start out in this race, only one (or two) will reach full maturity and release an egg. If you’re taking fertility drugs, several follicles may reach the ovulatory stage. The follicles themselves are responsible for:

  • Nourishing and protecting the oocyte as it goes through oogenesis
  • Releasing essential reproductive hormones
  • Transforming into the corpus luteum after ovulation, which releases the hormone progesterone (vital to keep the uterine lining thick and lush for a pregnancy to implant)

The Follicular Phase of the Menstrual Cycle

The follicular phase of your cycle begins on the first day of your period. Menstruation is the body's release of the top layer endometrial tissue, which was built up in anticipation of pregnancy. At the end of your period, the uterine lining will be thin. The lining will grow and become thicker again after ovulation.

But before that occurs, during your period, your ovaries are preparing the next egg for ovulation. Between five and six follicles will start to grow in your ovary. The hormone FSH—follicle stimulating hormone—is produced and released by the pituitary gland. It's this hormone that triggers the follicles to mature.

As the follicles increase in size, they release more estrogen. Higher levels of estrogen signal the pituitary gland to slow down the production of FSH. Even though you started with five or six follicles, just one (and sometimes two) will make it to maturity. Lower levels of FSH cause the smaller follicles to grow more slowly or even stop growing, while the bigger follicle continues its steady development.

Eventually, one follicle becomes the dominant follicle; the others stop growing and disintegrate. When the follicle reaches close to its maximum size, it releases even more estrogen.

Very high levels of estrogen trigger the pituitary gland to produce and release LH, or luteinizing hormone. This pushes the follicle to complete its last stages of development. Finally, the follicle will burst open and release an egg. This is the moment of ovulation.?

Folliculogenesis: The Stages of Follicular Development

You might think that follicular development starts and ends during the follicular phase of the menstrual cycle, but that is not the case. The full follicular lifecycle begins before a person is even born, when the ovaries are first developed in a growing fetus.

At this time, the ovaries contain only primordial follicles. Follicles can remain in this “sleeping” state for up to 50 years before waking up and going through the stages of development. It takes anywhere from six months to one year to go from a primordial follicle to a mature, ovulation-ready follicle.

At every stage of follicular development, many follicles stop development and die. Not every primordial follicle will go through each stage. Think of it as a competition to get to the Olympics of ovulation. Some follicles will drop out, and others will continue. Less than 1% ever actually ovulate an oocyte. The stages of folliculogenesis are:

  • Primordial follicle: The stage all follicles are in within the ovaries of a newborn baby
  • Primary follicles: A few primordial follicles move into the primary follicle stage every day, starting in puberty and continuing until menopause
  • Secondary follicles: Involves the addition of theca cells, which will secrete hormones
  • Tertiary follicles, also known as antral follicles: Follicles that contain a fluid-filled cavity called the antrum; follicles at this stage are visible via transvaginal ultrasound
  • Graafian follicle: A follicle large enough to ovulate; only one or two of the tertiary follicles in each cycle will mature to ovulation
  • Corpus luteum: No longer a follicle anymore; develops from the open follicle that released an egg

Follicle Size

If you’re going through fertility treatment, your doctor may monitor follicular development via ultrasound. During these ultrasounds, the number of developing follicles will be counted. They will also be measured.

Follicles are measured in millimeters (mm). Usually, your doctor will want to schedule your trigger shot—or hCG/LH injection—when your follicles are just about to reach full mature size. This is around 18 mm. A mature follicle that is about to ovulate will measure anywhere between 18 and 25 mm.??

How Many Follicles You Need for Fertility Treatments

The desirable number of follicles is different for various fertility treatments. You need more follicles for in vitro fertilization (IVF), for example.

Follicles Needed for a Clomid Cycle

Ideally, you only want one or two good size follicles during a Clomid cycle. You may feel disappointed when you find out only one or two follicles are big enough to ovulate. However, remember that more isn’t necessarily a good thing. Every mature sized follicle could release an egg, and that egg could become fertilized.

If you have two follicles, you could conceive twins. Or you might conceive one baby. Or, you might not conceive at all. Ovulation doesn’t guarantee pregnancy.

Follicles Needed for an IUI or Gonadotropins Cycle

As with Clomid, ideally, you only want one or two follicles to grow to maturity.?Injectable fertility drugs (gonadotropins) come with a higher risk of a multiple pregnancy. It’s possible to develop three, four, or even more mature follicles.

If you get four or more follicles, your doctor may cancel your treatment cycle. This may mean canceling a scheduled IUI procedure, canceling the trigger shot, and/or telling you to refrain from sexual intercourse.

If your doctor tells you not to have sex, it’s imperative that you listen. The risk of conceiving triplets or quadruplets is high with so many mature follicles. A multiple pregnancy will put your and your babies’ lives at risk. It’s better to wait and try again on another cycle.

Follicles Needed for an IVF Cycle

During IVF treatment, your doctor wants to stimulate your ovaries to mature several follicles. Anywhere between 8 and 15 follicles is considered an acceptable amount.

During an egg retrieval, your doctor will aspirate the follicles with an ultrasound-guided needle. Every follicle will not necessarily contain a quality egg. So don’t be surprised if the number of eggs retrieved is less than the number of healthy size follicles you were told you had.

A Word From Verywell

Follicle size and counts can be a source of stress. A diagnosis of low ovarian reserves can be especially difficult to cope with. Your doctor may recommend IVF with an egg donor, a path that not all couples are able or willing to take.

While follicle counts are an important indicator of fertility, remember that one number does not define you, or even absolutely predict your fertility future. If you're unsure what your follicle counts mean, talk to your doctor. Don't be afraid to seek a second opinion on fertility testing and diagnosis results.?And be sure to reach out for support. Fertility testing and treatment is stressful. You do not need to do this alone.

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