5 + 5 = 2? The Difference between Follicle Count and Embryo Number

Numbers are no strangers to fertility medicine; success rates, dosages, and egg/embryo counts are all things we count. And despite our tenuous personal relationship with math, over time, we have become quite comfortable with statistics, percentages, and probabilities (disclaimer—as they relate to IVF only)!

However, the number that often eludes us, and the question on so many patients’ minds, is the following: What is the follicle to egg to embryo to viable (able to make a baby) embryo conversion rate? Simply stated, if I have 10 follicles will I have 10 eggs, and if I have 10 eggs will I have 10 good embryos? The short answer, without any derivatives, formulas, or equations is no…You will very likely not. And here’s why.

Human reproduction is an incredibly inefficient process; think the DMV on a bad day! While we start with over a million eggs, a very small percentage of them actually have the potential of making a baby. So while a woman may ovulate every month, many of these months the egg that is released won’t put you on the path to pregnancy. Now, while in most natural un-medicated cycles only one egg is released per month, there are actually a group of eggs that are vying for the ovulation “trophy.”

Think of egg /ovulation selection as a horse race. At the beginning of the month, several horses (a.k.a. eggs) are racing to become the egg that will be ovulated. Ultimately, one breaks free, garners enough receptors to capitalize on the available hormones, and wins the ovulation race. When we are young, there may be 30 or 40 “horses” that enter this race. Although there will still only be one winner at the finish line, the race is more robust. As a result, there is a much higher chance that your winner will be fast, strong, and able to get the job done.

As we age, the number of “horses” lining up at the gate declines until we are left with only a few weaker, slower, scrawnier participants. There will still be a winner, but you may not get very many calls from interested breeders!

Let’s stay with the horse-racing metaphor for a moment. One of the primary goals of an IVF cycle is to ensure that all of the horses that start the race finish it; in this race, we don’t want a winner. As fertility doctors, we give hormones to ensure that there is enough juice to get every follicle/egg to go the distance and cross the finish line. We want all of the eggs in that month’s cohort to grow and develop at the same rate and ultimately achieve maturity.

However, even with the strongest of cocktails we are limited by the number of entrants. If five horses enter the race, we can have no more than five horses finish it. Think of the first ultrasound in an IVF cycle as the race check-in. If the doctor sees five follicles (remember, eggs are microscopic so we count follicles that hold the eggs) there are likely to be no more than five eggs extracted at the time of the retrieval.

Unfortunately, IVF is not as simple as horse racing, and the follicle number seen at the start does not always translate into the egg number you have at the end. There is a very large attrition rate as you move throughout an IVF cycle. The race is longer than a couple of laps around the track, and therefore, the numbers drop off quickly.

And the situation only becomes more complex as women age. As the years rise, the baseline follicle count falls, and you lose your cushion. With a diminished starting follicle count at the outset comes a decline in egg number at the conclusion. Fewer eggs equal fewer embryos. Fewer embryos mean fewer viable embryos.

No matter how young or old you are, there will always be a noticeable decline in the follicle to embryo equation. It is a function of human reproduction. However, if you are going to fall, you hope that there is a cushion below you to break that fall.