In the ovaries of the female reproductive system, an ovarian follicle is a fluid-filled sac that contains an immature egg, or oocyte. These follicles are found in the ovaries. 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. 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 percent 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.
What Are Antral Follicles and How Do They Predict Fertility?
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. They 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 finally visible on the 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 a woman has very few antral follicles developing on the ovaries, you suspect that her ovarian reserves are low.
Antral follicles produce higher levels of a hormone known as anti-mullerian hormone (AMH). This hormone circulates in your blood. Measuring AMH levels via blood work is another way to evaluate ovarian reserves.
What Is an Antral Follicle Count (AFC) Test?
An antral follicle count is a fertility test. It’s done via transvaginal ultrasound, sometimes between cycle day 2 and 5. The ultrasound tech will look at each ovary and count the number of follicles measuring between 2 and 10 mm.
Your doctor may order this test to
- Evaluate your ovarian reserves
- Make a diagnosis of primary ovarian insufficiency
- Help diagnose polycystic ovarian syndrome
- Give you an idea of where your fertility stands in relation to your age (do you have more, less, or an average number of follicles compared to others your age)
The test may be done as part of a fertility workup. Or, it may be ordered before a fertility treatment cycle.
How Many Antral Follicles Are Normal?
Remember that it is normal for your ovarian reserves to go down as you age. So, what’s normal for a 25-year-old isn’t necessarily normal for a 38-year-old. With that said, 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 center gets 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 a 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, including estradiol and inhibin B, which signal the pituitary and hypothalamus gland to increase or decrease the release of GnRH, FSH, and LH
- Transforming into the corpus luteum after ovulation, which releases the hormones progesterone and estrogen
What Happens During 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, as you’re on 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 and more estrogen. The 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. The lower levels of FSH cause the smaller follicles to grow slower or even stop growing, while the bigger follicle continues its steady development.
Eventually, one follicle becomes the dominant follicle. The others stop growing and die. When the follicle reaches close to its maximum size, it releases even more estrogen. While before, increased levels of estrogen led to decreasing levels of FSH, when the estrogen levels are exceptionally high, there is a switch in how the pituitary gland reacts to the hormone.
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, and 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 you would be wrong about that.
The full follicular lifecycle begins before a girl is born when the ovaries are first developed. 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 of the follicles will 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 one percent 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 girl)
- Primary follicles (recruitment of a few primordial follicles into the primary follicle stage occurs every day, starting in puberty and continuing until menopause)
- Secondary follicles (which involves the addition of theca cells, which will secrete hormones)
- Tertiary follicles, also known as antral follicles (which are 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 (not technically a follicle anymore, the corpus luteum develops from the broken open follicle that released an egg.)
How Big Should Follicles Be?
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 Should I Get During 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 not conceive at all. Or, you might conceive one baby. Ovulation doesn’t guarantee pregnancy.
How Many Follicles Are Normal for an IUI or Gonadotropins Cycle?
Like 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 you listen. The risk of conceiving triplets or quadruplets is high with so many mature follicles. A multiple pregnancy will put you and your babies’ lives at risk. It’s better to wait and try again on another cycle.
How Many Follicles Should I Have During an IVF Cycle?
During IVF treatment, your doctor wants to stimulate your ovaries to mature several follicles. Anywhere between 8 and 15 follicles are 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 Tips For Healthy Living
Follicle size and counts can be a source of stress. During testing, learning that your antral follicle count isn’t as high as hoped can cause worry. 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.
Follicle counts and measurements during fertility treatment can also send you on a roller-coaster of emotions. Are your ovaries producing too few? Too many? What does it all mean?
While follicle counts are an important indicator of fertility—and during treatments, follicle counts can determine whether a cycle will proceed or be canceled—remember that one number does not define you, or even absolutely predict your fertility future.
Women with low antral follicle counts can get pregnant, sometimes with their own eggs. One ultrasound during a treatment cycle may not bring good news, but an adjustment of your fertility drugs could make the next ultrasound results much better.
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.
- Antral Follicle Count. USC Fertility. http://uscfertility.org/fertility-treatments/antral-follicle-count/.
- Jayaprakasan K, Campbell B, Hopkisson J, Johnson I, Raine-Fenning N. “A prospective, comparative analysis of anti-Müllerian hormone, inhibin-B, and three-dimensional ultrasound determinants of ovarian reserve in the prediction of poor response to controlled ovarian stimulation.” Fertil Steril. 2010 Feb;93(3):855-64. doi: 10.1016/j.fertnstert.2008.10.042.
- Scheffer GJ, Broekmans FJ, Looman CW, Blankenstein M, Fauser BC, teJong FH, teVelde ER. “The number of antral follicles in normal women with proven fertility is the best reflection of reproductive age.” Hum Reprod. 2003 Apr;18(4):700-6. doi: 10.1093/humrep/deg135