Breaking It Down: Follicles, AFC, and Egg Maturity


3 min read

Starting In Vitro Fertilization is a process with a serious learning curve filled with new information and medical jargon. An IVF cycle is essentially stimulating your ovaries to grow eggs until they’re ready to be retrieved, but how will you know how many eggs to expect, how will the doctor know when they are mature, and why does maturity matter? The answers to these questions are crucial to understanding how IVF works and what outcome you can expect. 

What Are Follicles? 

Follicles are fluid-filled sacs that each house one egg. The follicles start off tiny and dormant. Each month, a group of follicles are brought toward the surface of the ovary and are visible on ultrasound. These are known as antral follicles. 

Antral follicles are very small and can be visualized at the beginning of your menstrual cycle on Cycle Day one or two. As the menstrual cycle continues and your hormones begin to shift, one follicle is selected to be the Dominant Follicle

In a normal menstrual cycle, the dominant follicle is the only follicle of the original group that grows in response to the hormones, while your body absorbs the rest. Conversely, in an IVF cycle, the ovarian stimulation medications that you take cause many more of these antral follicles to respond by growing in size and creating several mature eggs to be retrieved at the end of the cycle. 

What Is the Antral Follicle Count (AFC)? 

Your antral follicle count is one of a few values that help predict how you might respond during ovarian stimulation for IVF (along with an AMH blood test). Antral follicles are best visualized by a Transvaginal Ultrasound on cycle day one or two of your menstrual cycle. During the ultrasound, all of the small follicles in the cohort will be counted. This count, known as an Antral Follicle Count, is part of estimating your Ovarian Reserve. The count is similar each month, so it can be used to predict approximately how many follicles could possibly respond to the medications during your treatment cycle, keeping in mind that not all of the antral follicles will respond to the medications. 

Antral follicles are best visualized by a transvaginal ultrasound on cycle day one or two of your menstrual cycle.

Your AFC decreases with age as you approach menopause and the quantity of eggs in the ovaries decreases.

What Does Maturity Mean in Regards to an Egg?

The dominant follicle grows in response to the increase in circulating hormones. As the follicle grows, the egg inside is maturing. The maturation is crucial because an egg can only be fertilized at a certain stage of development known as Metaphase II or Metaphase II

At your ultrasound appointments, the doctor will measure each follicle in millimeters (mm). Follicles between 16mm and 22mm are expected to contain a mature egg. When planning the egg retrieval, your doctor will closely monitor your follicle growth until the majority of the follicles fall within this range. 

Follicles that are smaller than 16mm are unlikely to contain a mature egg, and follicles greater than 22mm will most often contain a post-mature egg. Neither immature nor post-mature eggs can be fertilized to create an embryo, so it’s important to have as many follicles as possible that fall within the preferred size range.

It’s not uncommon for the doctor to allow one or two follicles to become post-mature in order to allow a larger portion of follicles to reach at least 16mm and the point of egg maturity.

Together with the AFC, knowing the number of mature-sized follicles can help you and your fertility specialist plan out how many treatment cycles might be needed to create the optimal number of embryos to meet your family planning goals.

Will knowing the number of mature-sized follicles tell me exactly how many eggs will be retrieved?

In short, no. There is often a difference, higher or lower, between the number of follicles that were measured at your last ultrasound and the number of mature eggs that are actually retrieved. Unfortunately, there is no way to confirm the exact number of mature eggs until the eggs are examined under a microscope in the lab. Once they're analyzed by the lab, the mature eggs are fertilized to create embryos and the immature eggs are discarded.

When starting IVF, almost all of the words and concepts are foreign to most patients. Knowing a little bit about your follicles and eggs allows you to participate actively in your appointments and have an educated insight into what is happening within your body during treatment and the factors that influence your outcome.