How many eggs should I freeze?

I wish there were one number I could give you. Unfortunately, it isn't quite that simple.

Whenever I see a patient considering egg freezing, I hear the same two questions: How many eggs should I freeze? And can we collect enough in just one stimulation cycle?

To answer them, we need to add two more numbers to the equation: your age and your AMH level. They tell us two very different things. AMH helps us estimate how many eggs we might be able to collect during ovarian stimulation, while age is the strongest predictor of their reproductive potential — particularly the likelihood that an egg is chromosomally normal.

In other words, AMH helps us estimate how many eggs we may get. Age helps us understand what those eggs may offer.

First, not every egg retrieved during ovarian stimulation is mature enough to be frozen. What matters is the number of mature eggs, known as metaphase II (MII) oocytes, which are ready for fertilisation.

Even among mature eggs, however, reproductive potential varies considerably with age. As women get older, the proportion of eggs with chromosomal abnormalities increases, reducing the likelihood that each individual egg will eventually result in a healthy embryo and a live birth.

This is why ten eggs frozen at 28 and ten eggs frozen at 40 do not represent the same reproductive potential.

Of course, the number matters too. Freezing more mature eggs increases the opportunities for fertilisation, embryo development and ultimately a live birth. But the number needed to achieve a reasonable chance of success rises with age. And collecting that number often becomes more difficult as ovarian reserve declines.

We have predictive tools that help illustrate this. For example, the Brigham and Women's Hospital Egg Freezing Counseling Tool estimates that approximately 10 mature eggs frozen between ages 30 and 35 may offer a similar chance of at least one live birth as 35 eggs frozen at age 40. And here lies the paradox of egg freezing: as women get older, they need more eggs to achieve a reasonable chance of success, yet their ovaries tend to produce fewer eggs in response to stimulation.

These are mathematical estimates, not guarantees. Actual outcomes depend on several factors, including the experience and performance of the embryology laboratory.

So when counselling women about egg freezing, I prefer to start with age, then discuss numbers — not the other way around. What matters most to me is that each woman understands what her frozen eggs may realistically offer her in the future.

Now, coming back to your original question. Using the BWH Egg Freezing Counseling Tool, and assuming that you freeze 15 mature eggs at the age of 36, the estimated probabilities are:

• 74% chance of at least one live birth
• 38% chance of at least two live births
• 14% chance of at least three live births