Ovarian follicle definition
Ovarian follicles are small, fluid-filled sacks on the surface of the ovary. Each ovarian follicle contains a single egg (oocyte) and supports the egg through all stages of maturation. The number of ovarian follicles detected on ultrasound is sometimes used in fertility evaluations as one proxy of ovarian reserve, the ovary’s ability to produce healthy eggs that can lead to healthy pregnancies.
What are Antral follicles?
Antral follicles are the smallest ovarian follicles that are visible on ultrasound. As eggs inside the follicles “wakes up” and goes through the maturation process that eventually ends in atresia (cell death) or ovulation, the follicles grow in size. Resting follicles and pre-antral follicles are almost invisible at below 0.2 mm. Antral follicles are the first stage that is detectable on ultrasound. Once the follicles are at the recruitable stage (i.e., become candidates for full maturation and eventual ovulation), they are about 2 mm in diameter. Dominant follicles, ready for ovulation, can be about 2 cm.
Many colleagues, especially in Europe, frequently use the so-called antral follicle count (AFC) to assess functional ovarian reserve (FOR). As AFC denotes in its name, it represents an assessment of the so-called antral follicles in both ovaries. Although antral follicles are the smallest follicles visible on ultrasound, they represent quite an advanced stage of follicle maturation. Indeed, they are already beyond what is called the small growing follicle stage, where DHEA and human growth hormone (HGH) are effective in affecting the number and quality of the eggs.
The AFC has been demonstrated to correlate reasonably well with FSH as well as AMH levels in determining FOR. At CHR, our studies have, however, suggested that the sonographic assessments of AFCs differ too much between physicians. This does not mean that CHR does not look at AFCs and often even records them but CHR does not use AFCs to determine stimulation protocols for each patient. Instead, the reliance is on FSH, AMH, estradiol levels and very accurate follicle size assessments. The latter has in recent years gained additional importance because of Highly Individualized Egg Retrieval (HIER), which, considering CHR’s patient population, now a majority of the center’s patients are treated with.
Antral follicle count by age
Many patients ask us what the normal antral follicle count by age is. There are also some websites that give out the supposed average antral follicle count by age. In the medical literature, there have been attempts at defining the normal range as well. One studyof subfertile Chinese women, conducted in Singapore, found that 50thpercentile of antral follicle count is around 20 at age 30, steadily falling to below 10 by age 44. While the age-related decline in antral follicle count in general is incontrovertible—after all, ovarian reserve declines with age—attempting to define a “normal” antral follicle count by age is meaningless due to the above-noted variability in how physicians measure and count antral follicles.
This is a part of the November 2018 CHR VOICE.