How the body chooses an egg to ovulate or release is very complicated and we are just starting to understand how this process works. One in eight couples struggle with infertility for a number of different reasons – this is unique to humans and not found in any other mammal species! For animals, it is much more predictable and consistent. However, for humans this process is much more delicate and inefficient. Generally women are born with thousands of eggs. When puberty starts, the body recruits a group of eggs that could grow but usually only one is selected to be the one that is released and the others are no longer available for any future cycles. As a woman gets older, naturally the selection of eggs gets less over time and the group of eggs recruited each cycle is smaller. Understanding why some women are fertile into their 40s while others stop releasing eggs consistently each month in their 20s has been elusive.
There is no one perfect test for ovarian reserve. There is no test that can tell a woman how many eggs she has left or if the egg she is releasing any given month is a healthy one. Several tests are used routinely as indirect measures of ovarian reserve but it is a misnomer to call them tests of “ovarian reserve” because these tests do not tell us the actual quantity of eggs available. Some common tests that your doctor may order include baseline FSH and estradiol levels, AMH, and preantral follicle counts.
Follicle stimulating hormone (FSH) comes from a gland in the brain called the pituitary gland. Baseline FSH levels within a few days of the first day of menses give us an estimation of how hard this gland has to work to stimulate the ovaries to mature an egg. If it is low or less than 11, this is reassuring and suggests that the body should respond to fertility treatment to stimulate the ovaries. When it is high, this is less reassuring as the gland must work very hard to mature an egg.
Anti-Mullerian hormone (AMH) is produced by the small follicles that contain the eggs in the ovaries so the more follicles, the higher the AMH value. Having a high value is good (opposite from baseline FSH levels!). Likewise your doctor may count the number of follicles seen on ultrasound in the beginning of a cycle, or preantral follicle count, to gauge how many eggs the ovaries have recruited that month. When there is a healthy selection of follicles, there are more eggs to choose from that cycle.
Remember there is no one perfect test so your doctor will most likely use a combination of these tests to get their best assessment of your fertility. Most importantly, these tests do not indicate whether you can or can not get pregnant but your doctor will explain how they affect your fertility and treatment options. There are other tests for fertility and your doctor will recommend the tests that are most appropriate for each individual depending on the medical history. There are many treatment options available but understanding which ones are most appropriate or potentially successful for an individual depends in part on the findings from these standard ovarian reserve tests.