What is Ovarian Reserve and How Do We Evaluate It?
Ovarian reserve is a general term used to describe the capacity of the ovary to produce healthy eggs capable of being fertilized and ultimately leading to a live birth. The general term “ovarian reserve” is used to reference the eggs readily available for maturation and release, sometimes called the “functional ovarian reserve”. The functional ovarian reserve is really only one component of the total ovarian reserve, which includes all of the follicles that are non-growing follicles as well as the follicles that are growing (functional ovarian reserve). One of the ways we estimate the functional ovarian reserve in clinical practice is through the follicle count performed during vaginal sonography. The onset of menstruation (menarche) is viewed to be the starting point of follicle depletion. As women age, the number of follicles entering the maturation phase decreases. Fewer pre-ovulatory eggs are produced, and egg quality declines. The end result is reduced treatment success rates with various treatments including IVF.
The traditional method of evaluating ovarian reserve is the cycle day three serum FSH and estradiol determination. When ovarian reserve is diminished, the FSH or estradiol levels will be elevated. These laboratory tests provide indirect evidence that the ovary is underperforming. It suggests that the ovary contains a significant percentage of eggs that are unlikely to fertilize normally and lead to a live birth. It is important to realize that when these indicators (FSH and estradiol) are elevated in younger patients (under 35), pregnancy is still quite possible and treatment should be offered rather than declined. For older patients (40 and older), the significance of an FSH elevation is predictive of much poorer outcomes and patients should be counseled regarding low pregnancy rates in these situations before treatment is recommended.
Measurement of blood levels of AMH or anti-mullerian hormone is another method for assessing ovarian reserve. AMH is a substance produced by the cells that line the inside of ovarian follicles. The smaller growing follicles produce the greatest quantity of AMH. Studies suggest that the size of the growing group of eggs is highly reflective of the larger group of non-growing follicles (primordial follicles). Therefore the AMH level is felt to reflect the size of the remaining supply of eggs, also know as “ovarian reserve”. As might be expected, women with higher AMH levels will produce more eggs when stimulated during IVF treatment while women with lower AMH levels will usually produce fewer eggs in response to stimulation. AMH testing has not been in common use for nearly as long as FSH and estradiol. Therefore, interpreting AMH levels must be done with caution. We do not have absolute values of AMH beneath which no pregnancies occur. Even patients with non-detectable levels of AMH sometimes conceive successfully. The AMH test does provide us with an additional test that should be used in conjunction with other tests of ovarian reserve testing such as FSH, estradiol, and ultrasound assessment of “antral follicle count”.
Antral follicles are ultrasonographically visible actively growing follicles. These follicles are generally 2-12mm in diameter and may be readily seen and counted with standard trans-vaginal ultrasonography. The number of antral follicles on each ovary provides additional insight into ovarian reserve because the number of actively growing follicles is felt to be reflective of the size of the remaining pool of available non-growing follicles.
To summarize, proper evaluation of ovarian reserve requires that we measure the cycle day 3 FSH and estradiol levels, perform a careful assessment of the antral follicle count by trans-vaginal ultrasonography, and measure the measure the blood level of AMH. The results of these tests should provide an excellent assessment of ovarian reserve, which can be used to counsel patients regarding appropriate treatments and prospects for success.