Fertility and age
How does female fertility change with age?
Why female fertility changes with age, how egg quantity and quality differ, and what tests can and cannot tell you.
Women are born with all the eggs they are going to have.
But ageing is not simply a matter of gradually running out of eggs. The quality of the remaining eggs changes too.
That is why age is perhaps the most important predictor of fertility — even when periods are regular and you feel exactly the same as you did ten years ago.
Before birth, the ovaries contain millions of immature eggs. By puberty, considerably fewer remain, and eggs continue to be lost throughout reproductive life.
Only a tiny proportion will ever actually be ovulated.
As women get older, two things become important:
- the number of eggs remaining falls;
- the proportion of eggs which will produce a chromosomally normal embryo also falls.
The second point is particularly important.
There is no single age when somebody changes from “fertile” to “infertile”. The change is happening every day, in small amounts. And it’s not the same for every woman.
Fertility generally declines gradually through the 30s, with the decline becoming more significant from the mid-30s onwards and particularly during the late 30s and early 40s.
But these are population patterns, not deadlines for an individual woman.
One woman may conceive quickly at 39. Another may have difficulty at 31.
Age changes the probability. It doesn't predict exactly what will happen to you.
Regular periods usually suggest that you are ovulating regularly.
That's reassuring.
But they don't tell us how many eggs remain, whether the fallopian tubes are open, whether sperm is normal — or, importantly, whether an egg is chromosomally normal.
So it is perfectly possible to continue having regular monthly periods while fertility is declining with age.
As eggs age, chromosomal errors in the embryo from the egg become more common.
An embryo with an abnormal number of chromosomes may fail to implant, stop developing very early or result in miscarriage.
This is one reason both difficulty becoming pregnant and miscarriage become more common as maternal age increases.
It also explains why simply knowing how many eggs remain doesn't give us the whole fertility picture.
There is no blood test that can tell a woman whether she can or cannot become pregnant naturally.
An AMH blood test can give useful information about ovarian reserve — broadly, the remaining pool of eggs — but it does not directly measure egg quality. A reassuring AMH result can certainly be encouraging.
Age remains extremely important when interpreting fertility.
A pelvic ultrasound can provide additional information about the ovaries and uterus, but again it cannot give you a percentage chance of natural pregnancy.
Tests in the context of fertility are really about avoiding a situation where there may be an issue — such as a low egg reserve or uterine pathology — and you are simply unaware of it.
If you are trying for pregnancy, the usual advice is to seek help after about a year if you are under 35, and earlier as age increases or if there is another reason for concern. And you can ask questions before trying that long too. Sometimes doing a couple of simple tests early — such as an AMH test or a semen analysis — may save time for at least some couples.
Perhaps you're considering delaying pregnancy, wondering about egg freezing or simply want a realistic explanation of age and fertility.
Finding out more doesn't commit you to doing anything.
Sources and further reading
- NHS fertility information
- NICE fertility guidance
- HFEA information on age and fertility
Medical review
Medically reviewed by Dr Mahadeo Bhide, MD, MRCOG
Last medically reviewed: September 2026