Fertility after 35: Why egg quality matters as much as quantity

It’s a common misconception that if you’re a woman, your supply of eggs will run out by your early 40s. As consultant gynaecologist Mr Mahantesh Karoshi explains, quality matters more than quantity, which is why certain fertility treatments such as pre-implantation genetic screening and egg freezing are becoming increasingly popular.

Although women are born with a finite number of eggs, It’s a misconception that women run out of eggs by their late thirties or early forties.

A newborn female has 1-2 million eggs. At puberty this number is reduced to around 300,000 - 500,000. At age 37, she has 25,000 and at age 51, she still has 1,000 eggs left.

So shouldn’t this mean that getting pregnant at 40 or even 50 is easy? Surely you only need one egg to fertilise? Unfortunately, this is fiction.

It’s the quality of your eggs that matters

Both egg quantity (the number of eggs a woman has available for fertilisation also called as ovarian reserve) and egg quality (the eggs that are genetically normal) decrease as woman ages. This decrease starts to accelerate by age 35 to 37 and dramatically decreases after age 40.

A decrease in egg quality means that, although there are available eggs after age 35 and 40, a higher percentage of them may be damaged. This makes the chromosomes defective and unable to produce a normal pregnancy and a healthy baby outcome.

This means that despite still having several thousand eggs, most women are no longer able to become naturally pregnant by their mid-40’s.

What are my chances of getting pregnant?

It varies from woman to woman, and also depends on whether you have a pre-existing conditions such as polycystic ovarian syndrome or endometriosis.

In terms of the average woman, however:

• if you are in your 20s, the chance of getting pregnant is around 80%.
• if you are aged 30 to 34, the chance of natural pregnancy within one year is around 60%
• if you are aged 35 to 39, the chance of conception within one year is approximately 50%

However, this isn’t the only thing that changes as you get older. The chance of miscarriage or pregnancy loss also increases significantly:

• if you are in your 20s, the miscarriage rate is around 10%
• if you are aged 30 to 34, the rate is around 12%
• if you are aged 35 to 39, the rate is around 20%
• after 40 the rate is around 40%.

How much difference does fertility treatment make?

Fertility treatment is becoming increasingly popular, and there are more options now than ever before if you are having problems with getting pregnant. Some of the most popular options include in vitro fertilisation (IVF) and ovarian stimulation combined with intrauterine insemination.

Fertility treatments can make some difference, especially if you have specific problems that mean you can’t conceive normally. For some women trying to conceive through IVF, it can help to go through pre-implantation genetic screening, to make sure the eggs you’re using are healthy and likely to result in a successful pregnancy.

However, the biggest factor is still age. By some estimates, an IVF cycle is 30% successful in women under 35, but only successful in 9% of cases in women aged 40-42. Many fertility clinics do not treat women over this age because the chances of success are so low.

Read more: Fertility treatments

Should I freeze my eggs?

The idea behind freezing your eggs is that you preserve some of your healthy eggs from when you are younger, to use later on. Freezing eggs is an expensive process and tends to yield just a handful of healthy eggs, so the difference it makes is limited.

At the moment, we simply don’t know how it affects your overall chances of getting pregnant, because the number of people who have had their eggs frozen in the UK is so small.

Read more: Should I freeze my eggs?

On the whole, your age is still by far the biggest factor in your chances of getting pregnant. The good news is that technology is improving all the time – and the number of women having children in their late 30s and 40s is higher than ever.