Our clinic remains fully open. Find out more for Covid-19 information.


What are follicles & why are they important in IVF?

15th January 2016 in IVF

This information was correct at the time of publishing. It may not reflect our current practices, prices or regulations.

A close up of an ovarian follicle under a microscope

If you’re researching IVF treatment and its success rates, you’ll see that ovarian follicles play a major part in each and every IVF cycle. But what exactly are follicles, and why are they so important to whether your IVF cycle has a good chance of success?

What are ovarian follicles?

Ovarian follicles are small, fluid filled sacs in your ovaries. Inside each follicle is an immature egg, which grows and increases the size of the follicle as it grows. The egg is released when the follicle reaches the optimum size, this is when ovulation occurs. 

What happens to follicles during IVF?

In IVF, your ovaries are stimulated to make the follicles grow and produce mature eggs. Ultrasound scans will tell us when your follicles have grown to the right size, and how many follicles have grown. You’ll be monitored throughout stimulation to ensure that your ovaries are responding as they should and so that any changes can be made to your fertility drug regimen and treatment schedule if needed. 

When your follicles are ready and are at the right size – around 18-20mm – you’re given a trigger hCG hormone injection. This makes your follicles prepare to release the mature eggs. We time your egg collection procedure in our clinic theatre for just before this happens. 

How many follicles do I need for IVF?

Follicle response will vary, but it’s important to remember that if you have a good number of follicles it doesn’t necessarily mean that you have a higher chance of success. What matters most is the quality of the eggs produced. It only takes one egg to make a baby. This is why some women who only produce a few follicles still have successful IVF.

On average, the number of follicles produced in a cycle is around 10-12, but results vary dependent upon your personal response to stimulation and other factors such as your age and medical history. 

What happens if I produce too many follicles?

Ovarian Hyperstimulation Syndrome is a potential side-effect of ovarian stimulation. This is when your ovaries over-stimulate and produce too many follicles and mature eggs. We’ll be able to identify if you’re at risk of developing OHSS throughout your initial testing and screening process and take the necessary steps to minimise this during your treatment cycle. 

If you have any questions about follicles and IVF, speak to our friendly Patient Advisors on 0161 300 2737.

Last updated: 24th September 2019