How To Predict The Number Of Eggs In An IVF Cycle
Dr. Susanna Park | June 2020
Unlike men, who create new sperm every day, women do not create new eggs. Women are born with a lifetime limited supply of eggs (~1 million eggs). This supply is called “ovarian reserve.” As women get older, the ovarian reserve decreases. In most women, the ovarian reserve will be empty at ~50 years old, at which time the woman will no longer experience a menstrual cycle. This is called menopause.
To be able to predict the number of eggs in an in vitro fertilization (IVF) cycle, one must first understand how the ovary functions. Women are born with a limited ovarian reserve. It is impossible to know exactly how many eggs are in this ovarian reserve because this supply is microscopic so it cannot be visualized. Therefore, indirect markers are used to estimate how many eggs remain in the ovarian reserve. Three of the most common indirect markers of ovarian reserve are:
Follicle-Stimulating Hormone (FSH) – this is a hormone that is secreted by the brain. The most accurate time to measure blood levels of FSH (which is measured together with blood levels of estradiol) is during menstrual cycle day 2, 3 or 4 (menstrual cycle day 1 is the first day of bleeding). During menstrual cycle day 2, 3 or 4, serum blood levels of both FSH and estradiol should be low. Normal FSH<10 mIU/mL. When FSH levels are >10 mIU/mL, it signifies that the ovarian reserve is low. This means that less eggs will develop in an IVF cycle.
Anti-Mullerian Hormone (AMH) – this is a hormone that is secreted by the eggs in the ovarian reserve. As the ovarian reserve decreases, the AMH levels decrease. The “normal” level of AMH depends on the age of the woman. A younger woman would be expected to have more eggs in the ovarian reserve and therefore higher AMH levels. Conversely, an older woman who would be expected to have less eggs in the ovarian reserve and therefore lower AMH levels. The higher the AMH level, the greater the number of eggs in an IVF cycle.
- Antral Follicle Count (AFC) – Every month, inside the ovaries, a group of eggs from the microscopic ovarian reserve begins to grow at least 2mm. This group of eggs is called antral follicles. Because these antral follicles are at least 2mm in size, they are visible by ultrasound. The number of antral follicles that is visible correlates to the number of eggs remaining in the ovarian reserve: the more antral follicles visible by ultrasound, the more eggs left in the ovarian reserve; the less eggs visible by ultrasound, the less eggs left in the ovarian reserve. Every month, one follicle from the antral follicles naturally grows big. This one follicle has a mature egg inside that will ovulate. Therefore, naturally (without fertility medication), women ovulate only one egg every month. Meanwhile, the other antral follicles that grew at least 2mm in size do not re-enter the ovarian reserve but rather, they die inside the ovary. However, if a woman uses fertility medication, the fertility medication stimulates the growth of the antral follicles that grew at least 2mm in size and prevent them from dying inside the ovary. This means that with the use of fertility medication, multiple antral follicles grow big and multiple eggs can ovulate at one time. Therefore, by knowing the antral follicle count, one can predict the anticipated number of eggs in an IVF cycle.