Follicles and Egg Yield: All You Need to Know

Every egg-freezing cycle begins with counselling, and prepping the medications with the aim of growing and freezing multiple eggs. 

This is followed by a series of consequent ultrasound scans that (kinda) go like this: 

"We've got 3-4 follicles growing"

"5-6 now, and some are at 4-6mm, and the rest are growing fast"

"We've got about 10 follicles! But 5 are above 16mm in size"

And when the egg retrieval finally happens, you get to know that you got 5 eggs. 

‍But wait, weren't there 10 follicles to begin with? 

So, what happened to the rest? Why don’t follicles and egg numbers match? 

Well, the science behind it is exactly what we explore in this blog post! Read on!

The difference between follicles and eggs

Let's nerd it out here for a second🤓. The ovaries contain millions of follicles at birth. But these are dormant (just snoozing away) until you hit puberty. Once your menstrual cycle starts, some follicles are selected every month to grow, and only one, dominant follicle gets ahead of the others and releases one mature egg, every month. The rest? Think of them as martyrs who slowly undergo cell death. 

Basically, follicles are fluid-filled sacs in the ovaries that house eggs, grow, and help them develop. 

On the other hand, eggs are the female gamete, carry genetic material, and only a mature egg can be released by the follicle and fertilised by sperm. 

Why the number of follicles and eggs don't always match

During an ultrasound, you and your healthcare provider may see a certain number of follicles, but the final retrieved egg count may differ. Here are two common situations that occur:

1. You were told a fewer follicle number but ended up with more eggs during retrieval.
2. You were told a higher follicle number but ended up with fewer eggs during retrieval.

The fact that these are common tells us that it is in fact a rare situation, where the number of follicles exactly matches the number of eggs retrieved. And if you’re searching for a specific reason behind it, here’s the truth: It depends on the growth of the follicles in the ovaries and how mature the egg cell is, once it's retrieved.

Let’s dive deeper into it, with the next section.

What are the factors that affect egg yield from follicles?

There are several factors that explain the difference in your follicle count vs your egg count. Here's a breakdown of a few: 

1. Response to medication - Egg freezing cycles involve medication to boost the number of eggs retrieved, but response to medication differs with each person. So how you respond to medication, is what dictates how many follicles grow and develop in your cycle 
2. Total number of “mature” eggs retrieved- For instance, if the total number of follicles even on the day of egg retrieval is estimated to be 10, you may end up with 5 mature eggs, owing to the rest of the follicles containing immature eggs. However, in unique cases such as IVM+ egg freezing, these immature eggs are kept in growth media until they're mature enough to be frozen too! 
3. Empty follicle syndrome- A very rare genetic syndrome, where seemingly normal follicles on ultrasound do not yield any eggs 
4. Hidden follicles- some small follicles may often miss the doctor's eye, on ultrasound. These may grow and develop to yield mature eggs.
5. Follicles with cyst- Some follicles may seem to grow to the normal size but may be filled with cyst and fluid instead²
6. Delayed egg retrievals - In the case of delayed egg retrieval or a follicle maturing earlier than the retrieval, the egg count can be less than estimated (in some cases, they're in the pouch of Douglas, a small organ near the ovaries! Sometimes, they can be retrieved³ but in other cases, a second cycle is done)