How Many Eggs?

How many eggs am I going to get? This is a good question and nearly every IVF patient will ask me this at some point during their treatment. While I've been doing IVF for a decade, I still stammer when trying to answer this question. Why don't I know how many eggs a woman is going to get in IVF when I am looking at the egg sacs (aka follicles) on ultrasound?

Let me explain. In a normal menstrual cycle, one follicle (containing one egg) grows. The follicle gets bigger and bigger until it ruptures at ovulation, releasing the egg from the ovary. The egg then heads down the Fallopian tube. During IVF we give women medications that make the ovaries grow more than just one follicle. We monitor a woman's ovaries to see how many follicles/eggs are growing and how fast they are growing. This monitoring of the follicles/eggs is done using ultrasound and blood estrogen levels. The ultrasounds are done every 1-4 days to assess the size of the follicles. We know when follicles reach a certain size, typically above 17mm, the egg is ready to be retrieved for fertilization.

Follicles do not all start at the same size nor do they all grow at the same speed so IVF ovary will have follicles of many different sizes. For most women follicles between 15-22mm in size will give us a mature egg 80% of the time when we perform an egg retrieval procedure. Some women will only have mature eggs from larger follicles and some will have mature eggs from smaller follicles. No one is the same. So we have variable-sized follicles, with variable maturity eggs inside, and not all follicles will "give up" their egg during an egg retrieval.

This is an image of an IVF ovary. You can appreciate the variability in the follicle sizes (the follicles are the black circles):

It is quite hard to predict just how many eggs a women will get at an egg retrieval. I can give an educated guess considering the ultrasound findings and the estrogen levels, but it's a guess nonetheless. So, when patients ask me how many eggs so I expect, I "hum and ha" a bit. I am afraid that if I overestimate the number, I will set them up to be disappointed with the egg yield, and if I underestimate the number and they get a higher count, they will lose confidence in my ability to predict outcomes. Ultimately, I give a good guess with a few disclaimers.