When does your biological fertility clock start ticking?

More and more of us are putting off having kids to focus on our careers, so much so that we now delay having a baby for longer than women in any other country. And with celebrities like Nicole Kidman and Mariah Carey giving birth for the first time in their forties, it’s easy to imagine that we have all the time in the world. But when does the female biological clock actually start to tick? We’ve spoken to top fertility experts to give you the lowdown.

British women are becoming mums for the first time later and later, and the birth of the royal baby earlier this month highlighted this trend. While Kate Middleton, at 31, was in no way old, she was still over a decade older than Princess Diana was when she gave birth to Prince William. Things have changed a lot in a generation and the number of first-time UK mothers in their 40s has risen by 15% in the last five years.

But at what age do we actually need to start worrying about having kids from a biological perspective? While lots of women are able to conceive later on, could some of us be at risk of missing our chance to become a mother if we wait for too long?

35 could be key

‘There is no “magic number” at which female fertility declines but we know that eggs deteriorate with age,’ says Infertility Network UK’s deputy chief executive Susan Seenan.

‘The speed of that deterioration will vary but rises more steeply after the age of about 35. The more “fertility aware” you are the better, so you can make an informed decision and be aware that it might take you longer to conceive,’ she adds.

The NHS agrees that 35 is a key age when it comes to female fertility. Women are most fertile in their early 20s and their fertility declines with age. From the age of 35, this fall becomes steeper.

‘Women in the 19-26 age group have double the chance of conceiving each menstrual cycle compared with 35-39-year olds,’ explains Fertility UK fertility nurse specialist Jane Knight. Women over 35 are also less likely to become pregnant from fertility treatments like IVF, and are more likely to suffer from miscarriages.

So does this mean that we all need to get pregnant by 35 or risk never being able to have kids? Not necessarily. You may want to start worrying when you reach 37, though. ‘Egg quality diminishes significantly from about 37,’ explains Jane.

Ask your mum

Your personal danger age could also be down to genetics. Fertility expert Zita West suggests looking at your mum. ‘One important question to ask is at what age your mother had the menopause as this may be the same for you,’ she says. ‘The decline occurs when the quality of the eggs is diminished with age, and this is usually in the late thirties to early forties.’

But just because you’re still having periods, it doesn’t mean that you’ll get pregnant easily. ‘Nature plays a cruel trick in that women tend to go on having periods long after they have ceased to ovulate,’ says Jane. ‘The ovulation mechanism becomes faulty and the egg quality is very poor long before a woman reaches her menopause, and a woman may no longer be fertile for up to ten years before she has her last period. The average age for menopause is about 51, but many women will have a much earlier menopause.’

So does that mean that, for the average woman, 41 could be the pivotal age?