It’s an issue that has faced women for several decades now, particularly as they have become increasingly independent and professionalised. The balance between advancing your career and deciding what age you would like to start a family.

For many women, the desire for career progression during their 20s leads them to defer the decision to have children.

For some, their 20s are the years in which they want, much like men of the same age, to explore the world, to live a relatively carefree life, to enjoy their youth.

Most people in their 20s are focused on travel and careers

Of course, it is completely a matter of choice. But in making their choice, as Professor Mary Wingfield outlined on Today with Sean O'Rourke, women should never lose sight of certain biological realities.

"Nobody wants to hear this message. Particularly women. But if you ask any fertility specialist how would you improve fertility worldwide, it’s to try to get us back to having children when we are younger."

Mary Wingfield’s new book is entitled The Fertility Handbook and was described by presenter Sean O’Rourke, as an up-to-date guide for anyone who wants to maximise their chances of pregnancy. And, if having children is on the cards for any woman, it is crucial she is fully informed when making life choices.

"If you are 35, you have half the chance of getting pregnant as somebody who is 25."

"If you are 40, it’s lower again. Nobody wants to hear those figures, they are dreadful.

"If you want to have a 90% chance of having three children, you need to start trying to get pregnant when you are 27."

If you want to have a 90% chance of having three children, you need to start trying to get pregnant when you are 27.

As you progress through life, there are blood tests you can do, estimating how many eggs you have left.

Other factors, without a doubt, play into the fertility of both men and women. But for women, the message from Mary is stark: "Knowing your age is the most important thing. If you are 30, you need to start thinking about it very soon."

As many as one in six couples experience difficulty conceiving, and as one of Ireland’s leading fertility experts, Mary Wingfield has helped many of these couples conceive over the course of her career.

In her new book, she explores many of the issues that impact on male and female fertility, apart from just age.

"If men have had serious injuries to their testicles, they might have fertility problems in the future" she said.

"If women have had surgery for a very bad appendicitis, if their mother had an early menopause, these are things to watch out for."

Being overweight is really bad for fertility. Being underweight is also bad. Moderation in terms of lifestyle is very important.

Smoking is a definite no-no.

And, if you want to maximise your chances of having children, you definitely need to moderate your alcohol intake.

Crucially, all of these things apply to men as well as women. Including, it seems, the age factor.

Being over or under weight is bad for fertility

"Once men go over the age of 40, it reduces their fertility by about 50%", said Mary.

These days, she said, many men are going to the gym, taking testosterone and other steroids, to make them look really good.

This behaviour "devastates the sperm count."

Stress is another factor but in an inverted sense. "Stress doesn’t cause infertility. But infertility causes stress… If you are so stressed that it stops you from having sex, or going for fertility treatment, that can be a factor."

Fertility treatments like IVF are only available in Ireland to those who can afford it, according to Mary Wingfield.

It is not available through the public health service, a situation described by Prof. Mary Wingfield as "a total disgrace".

IVF is only available to those who can afford it

The proceeds from the royalties from her new book are going to the Merrion Fertility Foundation, a group that aims to give fertility treatment to couples who cannot afford it, because of this lack of public funding.

Listen to the full informative interview with Prof. Mary Wingfield above.