Women are born with 1–2 million potential eggs, but that’s all the eggs they’ll ever have; unlike skin cells or blood cells, which regenerate, our bodies aren’t able to make more egg cells. So our ovarian reserve is like a “bank” that doesn’t take deposits—and we’re withdrawing from it with each passing month. By puberty, a woman might have 1 million eggs; at 25, maybe 300,000. Then, around 35, the decline starts to get a bit steeper, meaning she could lose an even higher number of eggs per month until menopause.
There are two good ways to measure egg quantity: an antral follicle count and an AMH (or Anti-Müllerian Hormone) test. During an antral follicle count, which is most meaningful at the beginning of a woman’s menstrual cycle, a doctor uses ultrasound to count the “activated” follicles, and uses that number to estimate her total ovarian reserve. AMH is a protein hormone produced by special cells inside the follicles, so the level of AMH in the blood can help doctors estimate the number of follicles inside the ovaries. Because AMH levels stay basically stable throughout a cycle, the blood test can be performed at any time. A typical AMH level for a fertile woman is 1.0–4.0 ng/ml, but, depending on age, many women will be higher or lower than this range.
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When we’re young, our ovaries are full of egg follicles, waiting to mature; we call this an egg’s “primordial” or “sleeping” state. During each menstrual cycle, a certain number of these follicles are activated to prepare for ovulation, but just one egg takes center stage to mature and be released from the ovary. The understudy follicles, on the other hand, are “reabsorbed” by the body (and effectively lost). Doctors estimate that while our bodies may naturally ovulate only 400 times in our lives, we lose upwards of 1,000 follicles—potential eggs—per month, and that loss accelerates as we get older.
During egg freezing, we use medication to prompt those otherwise lost eggs to mature, allowing your body to produce multiple eggs in one cycle. Egg freezing doesn’t lower your body’s natural egg supply; it simply makes use of more of the eggs you already have.