Your number of eggs is your ovarian reserve. That’s a measure of quantity, not quality, and of course quality matters too. But when you have a good ovarian reserve, you most likely will have some good quality eggs in there.
You were born with millions of immature eggs. Most of the immature eggs are snuggled inside microscopic follicles (small baskets of cells) that will lie quietly for months and years at a time. But every cycle during your fertile years, there will be some follicles that are primed to grow. These primed follicles are known as antral follicles.
Ideally, you will have a good-sized pool of these antral follicles each month.
Why do antral follicles matter?
Actually, if you are trying to get pregnant naturally, they don’t matter very much at all. After all, up until menopause, your body will generally find 1 egg a month from the pool to mature and ovulate. I’m not saying ovarian reserve doesn’t matter with natural cycles at all, but we’ve all seen natural conceptions in women who have very low reserves.
However, if you want to access fertility treatments, a good ovarian reserve is extremely helpful. For example, in IVF, we find our best pregnancy rates occur when we generate 3-5 high quality embryos. The best predictor for this outcome is 10-15 eggs at retrieval. (It is possible to have 3 good embryos from 3 eggs, but less likely). And the best predictor for 10-15 eggs is a good ovarian reserve.
How do we test your ovarian reserve?
Antral Follicle Count (AFC). The AFC is an ultrasound test. The ultrasound tech counts your antral follicles. The ideal AFC is 15-20 over the two ovaries. If your AFC is <10, your ovarian reserve may be low (assuming the tech is counting accurately). As a test, AFC is very much technician-dependent: not every ultrasonographer can measure AFC well, and there tends to be a lot of inter- and intra-observer variability (i.e. everyone measures a different number). If you are a bit gassy (or a bit overweight), it can be difficult. Our newest 3D ultrasound machines have the ability to record AFC’s automatically. However, the technology isn’t perfect, and we still prefer the accuracy of our clinical team to that of the computer.
Anti Mullerian Hormone (AMH). AMH is a newer blood test, and in my opinion, the most accurate test of ovarian reserve. AMH is a hormone released by cells that are involved with the growth of antral follicles. AMH levels correlate with the number of active antral follicles present; the higher the antral follicle count, the higher the AMH levels. I trust it more than AFC, because it seems to correlate with the number of active follicles. AMH can be tested through a regular blood test. It can be drawn during any day of the menstrual cycle whether or not you are on the birth control pill. In my opinion, AMH is the single most helpful test for women looking to understand their own fertility. That said, if you get an unusual number, it is reasonable to repeat the test. AMH can be difficult to process in the lab, so the occasional incorrect number will be generated. Ask to do it again if you are making important decisions based on AMH.
FSH. FSH is a blood test, and is the traditional test of ovarian reserve, perhaps because it was more accurate than ultrasounds used to be. These days, ultrasounds are incredibly accurate, and AMH has taken over as the more accurate blood test. FSH is the hormone that drives your antral follicles to grow. If you have a good ovarian reserve, your body doesn’t need to make very much FSH to start the process of egg maturation. A number less than 10 IU/L is good; less than 8 is ideal. Greater than 12 is a worry. FSH levels change month-to-month, as the number of antral follicles change. FSH levels are brought lower by the presence of estrogen, so your FSH level is only an accurate indicator of your ovarian reserve when Estradiol levels are <200pmol/l. Estrogen is lowest on day 3 of the cycle, which is why we usually measure FSH on day 3.
Tests of ovarian reserve are important, but they aren’t your only measurement of future success. They are, however, a very helpful guide to optimizing and personalizing your fertility treatments.