Follicles and Fertility
My colleagues and I occasionally field questions from patients about what fertility physicians are looking for in conducting an antral follicle count. This is a fascinating topic and my colleague at Pacific Fertility Center, Isabelle Ryan, MD, has been kind enough to share her expertise on this issue.
Women are born with all of the eggs (oocytes) that they will ever have. This is a set number, which is determined before birth. This pool of eggs is never replenished. A female fetus will have the greatest number of eggs around 16-20 weeks of pregnancy (6-7 million); at birth this number decreases to about 2 million; and by puberty to about 300,000. This constant and dynamic process of decline continues until menopause and is not interrupted by birth control pills, pregnancy, or ovulation. From this reservoir of eggs, fewer than 500 eggs will ovulate during a woman’s reproductive life.
There is a continuous process occurring in the ovaries, where eggs are constantly being prepared for the maturation process. It takes 3-6 months for eggs to develop and mature. As the eggs are developing, they transition from a primordial, to preantral, to then antral follicle. Antral follicles are visible by vaginal ultrasound. Antral follicles therefore represent the reserve of eggs in our ovaries and those that are candidates for selection and growth by fertility stimulation medications (gonadotropins).
When assessing one’s ovarian reserve (potential for a successful pregnancy), a number of parameters are evaluated. One of these is called the “antral follicle count” (AFC). An antral follicle count is typically done during the 2nd-4th days of menstrual flow, though it can probably be as accurately done during other times of the menstrual cycle. Studies show that the AFC is predictive of the expected ovarian response to gonadotropins. An AFC less than 6 total (between both ovaries), predicts a poor stimulation response. For those undergoing IVF, a similarly low AFC will be associated with a higher cancellation rate. As women approach their 40s, and as day-3 FSH results rise above 10 mIU/ml, this typically correlates with fewer eggs overall in our ovaries, and therefore a low AFC. Indirectly, a low AFC can correlate with diminished ovarian reserve.
In the same way that there can be monthly variability in day-3 FSH test results, there can be monthly variability in the AFC. More variability is observed in the AFC of young infertile women than in older women. However, overall a single AFC is still quite predictive of ovarian response under gonadotropin stimulation, and there is fairly good agreement between repeated AFC over consecutive cycles. In conclusion, doing an AFC is an adjunct to the day 3-FSH test to predict ovarian reserve and ovarian response to fertility medications.