January 29, 2024
6 Minutes

Follicles and Egg Yield: All you need to know

Follicles and Egg Yield: All you need to know
Written by
Navya Muralidhar
MSc Clinical Embryology & Embryologist
Amilis makes fertility digestible, accessible, and affordable to help you take charge of your reproductive health and live on your own timeline.

In an eggshell...

  1. In most cases, the number of follicles does not exactly correlate with the number of eggs retrieved 
  2. This can be due to follicle-related factors as well as egg-related factors 
  3. To estimate the number of eggs you may retrieve, it is essential to check follicle growth and size, and time the trigger and retrieval accordingly

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)

Can I predict egg yield with a specific follicle number?

To be honest, there's no simple math around this. But rather, there are some signs and features of a follicle that you can note down, for a more accurate guess on the number of eggs retrieved. 

A follicle is more likely to yield a mature egg if: 

  1. It is 16-22mm on the day of retrieval4
  2. If it is clearly visible on ultrasound 
  3. If its growth is "normal" according to the stimulation cycle

And if you make a guess based on the number of follicles present, the number of eggs is more likely to co-relate if: 

  1. The trigger injection to mature the eggs is given at the right time 
  2. If egg retrieval happens at the right time

Another thing to note is that, If you've had a previous record of responding well to the hormonal medication for egg freezing, you can gauge your response and number of eggs more accurately for the current cycle.

How many eggs are typically collected from 15 follicles?

Now let's put these estimations into action. Assuming that the follicles meet all the criteria mentioned above, and if the trigger injection and egg retrieval are also timed well, here's an estimate: 

It's still very rare that 15 follicles lead to 15 exact, mature eggs. The number is most often less than the actual number of follicles estimated. So for 15, it's best to aim around 8-10 or 12 at the most. 

Amilis's take on egg yield during your egg-freezing cycles 

Egg yield during your egg-freezing cycles does not directly correlate with the number of follicles. But oftentimes, rather than the number, the quality of the egg is key. 

It is okay to feel disheartened with a lesser egg yield when you were reported with a higher follicle count. But knowing this difference in egg yield and follicle number beforehand can help you mentally prepare, gauge, or estimate cycle outcomes better. It also helps healthcare providers counsel, and plan cycles accordingly. 

We at Amilis feel that this knowledge is what empowers us, and you alike, to make informed decisions for ourselves. What also helps is the right support, guidance, healthcare providers, and clinics that help turn those decisions into reality. 

If you're looking to move forward with egg freezing as an option, your guide to moving forward with the right resources can be Amilis. You can get an overview of the costs with our clinic comparator, which compares UK's best, pre-vetted fertility clinics. You can also schedule a free consultation call with an Amilis expert, or take our personalised quiz to start your journey.

We’re here to support you all the way.

Written by
Navya Muralidhar
MSc Clinical Embryology & Embryologist

An embryologist by degree, and an educator by heart, Navya has completed her Bachelors in Genetics, and her Masters in Embryology and now strives to deconstruct the complex, into educational and informative articles surrounding her field of interest. She's specifically focused on time-lapse technology, IVM, and pre-implantation genetics. When not writing, you can find her at her favourite or newest coffee shop in town, sketching away, or listening to a podcast.