Image copyright Thinkstock Image caption The female fertility window can start closing from the age of 35

TV presenter Kirstie Allsopp has urged women to put off higher education and a career in favour of having children because their "fertility falls off a cliff".

In a recent interview with Jeremy Paxman on BBC Newsnight, Ms Allsopp, who met her husband when she was 32 and then had two children, said: "Nature is not with you and I. Nature is not a feminist."

She also encouraged women to be "more honest" with one another about their biological clock, saying the topic was still "taboo".

It is tempting to want a black and white answer, but biology doesn't work that way Mr Yacoub Khalaf, Guy's and St Thomas' Hospital

So what is the truth about the female fertility window?

Guidance from the National Institute for Health and Care Excellence (NICE), which was updated in 2013, is pretty encouraging.

It says that "over 80% of couples in the general population will conceive within one year if the woman is aged under 40 years", if they have regular sexual intercourse and do not use contraception.

NICE guidelines go on to state that "of those who do not conceive in the first year, about half will do so in the second year".

That leaves around 10% of women - the percentage said to be affected by infertility in the UK.

Image copyright PA Image caption Kirstie Allsopp questioned whether women should prioritise further education and work before their fertility

We know, of course, that female fertility declines with age, but is there really a dramatic drop-off at a certain point?

No rule

Mr Yacoub Khalaf, head of assisted reproduction at Guy's and St Thomas' Hospital in London, says it is not quite as simple as that.

"It is tempting to want a black and white answer, but biology doesn't work that way.

"Some women find it difficult to conceive in their late 20s, while others don't have a problem into their 40s."

Fertility facts Infertility is when a couple cannot conceive despite having regular unprotected sex. It can be caused by a range of disorders and lifestyle factors. However, fertility problems cannot be explained in around a third of cases. What affects your fertility?

So there is no rule, but science tells us that a woman's body does gradually change and there is nothing that can be done to alter that process.

Mr Khalaf explains that from the age of 35, the rate of depletion of the follicles in the ovaries speeds up, and from the age of 40 they start to deplete even faster.

These follicles are important because they house the eggs which will develop and mature before finally being released during ovulation.

Hence, the quality and quantity of a women's eggs also begins declining sharply from around the age of 35 onwards.

Disappearing eggs

It is a very different story at birth. Baby girls are born with a finite number of eggs, which can number around one million.

By the time of their first period, however, only 400,000 eggs will be left and they continue to decline in number throughout adulthood at a rate of approximately 1,000 eggs each month.

We tend to always talk about IVF success rates - but we don't say it's unsuccessful for most women Alison McTavish, University of Aberdeen

"They are much more accessible in the early years," Mr Khalaf says, referring to women's eggs in their mid-20s.

"I would rather have women trying for a baby as soon as they can because they will have healthy eggs, a healthy pregnancy and the energy to enjoy their baby."

But he recognises that the realities of life mean this is not often possible.

Higher education, career, finding Mr Right - all mean that women may not start thinking about having children until well into their 30s.

By that time, it is possible they may run into problems.

Seek help

Infertility Network UK advises women and their partners not to be complacent about fertility problems and to seek help from their GP.

This is because there may be gynaecological disorders which women are unaware of which could come to light.

These include polyps or fibroids, endometriosis or pelvic adhesions, which can be treated to maximise fertility.

Image copyright Thinkstock Image caption Careers often come before children for many women

As women reach their 40s, their risk of miscarrying increases to nearly a third of all pregnancies. The chance of giving birth to a baby with Down's Syndrome also rises significantly.

Alison McTavish, nurse manager at the University of Aberdeen's assisted reproduction unit, says when women are already on the "slippery slope" between 30 and 34 years old they mistakenly look to IVF as being a solution.

"This sometimes gives them false hope - we're not that good though.

"We tend to always talk about IVF success rates, but we don't say it's unsuccessful for most women."

After 40, there is a 5% chance of a woman becoming pregnant without IVF, increasing to 10% with the help of IVF, she says.

Fertility experts agree that the female fertility window has not changed much over the decades. The menopause still occurs in the same age range as it did for our mothers and grandmothers.

What has changed though is the male sperm count, which has been decreasing over the years.

The reasons for this are not known, although there are theories, so perhaps women should remember to keep check on their partner's fertility as well as their own.