Understanding the Ovarian Follicle

What It Is, What They Do, and How Many Should You Have

Microscoptic image of a primary follicle
A microscoptic image of a primary follicle in a human ovary. Jpogi / Wikicommons / CC BY

In 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.

The follicles that do not release a mature egg disintegrate and this can happen at any stage of follicular development.

This is known as atresia.

About 99% of ovarian follicles will disintegrate and never become mature enough to release/ovulate an egg.

After ovulation, the follicle that releases an egg turns into a corpus luteum.

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.

What Are Antral Follicles?

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.

Antral follicles – a follicle in a specific stage of follicular development – can be counted via ultrasound technology. An antral follicle count may be part of fertility testing.

The number of antral follicles can give your doctor an idea of how many viable eggs are left in your ovaries.

This is one way to evaluate ovarian reserves.

Antral follicles produce higher levels of a hormone known as Anti-Mullerian Hormone, or AMH. This hormone circulates in your blood.

Measuring AMH levels via blood work is another way to evaluate ovarian reserves.

What Role Do 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 actually reach full maturity and release an oocyte, or 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 GnRH, FSH, and LH
  • Transforming into the corpus luteum after ovulation, which releases the hormones progesterone and estrogen

Get a much more detailed explanation of the menstrual cycle here: How the Female Reproductive Cycle Really Works

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 like a competition to get to the Olympics of ovulation. Some follicles will drop out and others will continue on up. 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 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 known as 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 very important 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 is considered a good 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.

More on ovulation:

Sources:

27.2 Anatomy and Physiology of the Female Reproductive System. OpenStax College. Rice University. Accessed February 7, 2016. http://cnx.org/contents/FPtK1zmh@6.27:nMy6SWSQ@5/Anatomy-and-Physiology-of-the-

Erickson, G, Glob. libr. women's med., (ISSN: 1756-2228) 2008; DOI 10.3843/GLOWM.10289. Follicle Growth and Development. Accessed February 7, 2016. http://www.glowm.com/section_view/heading/Follicle%20Growth%20and%20Development/item/288

Hunter, R. H. F. Physiology of the Graafian Follicle and Ovulation. Cambridge University Press; 1 edition (June 9, 2003). ISBN-10: 0521781981. https://books.google.com/books?id=aCA-RKkr3wAC&pg=PA195&dq=follicle&hl=en&sa=X&ved=0ahUKEwib6rGiuNzKAhVV7WMKHaQ1BK4Q6AEIIzAB#v=onepage&q=follicle&f=false

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