We often hear women saying that their biological clock is ticking as an indication that they understand that child-bearing should not be postponed indefinitely. Perhaps it is wisest to view the years between 30 and 35 as time to start the ‘late family’ rather than 35 if at all possible.

Life is busy. Couples marry later and defer having their children because of careers and finances. Sadly, this seemingly perfectly reasonable decision often has unfortunate consequences. Both men and women become less fertile as they age. The facts are indisputable and at least for the moment there is no easy solution. For women, there is a stage that is sometimes called ‘oopause’ that usually occurs about 8-10 years before menopause. Oopause is the stage at which eggs have very low fertility and a very high chance of development being arrested and a pregnancy miscarrying.

From 1989 to 1993, my colleagues and I conducted a research project which we called ‘PALS – Pregnancy and lifestyle study’. Put simply, we recruited 600 couples who were planning a family, recorded their answers to a large number of questions, performed some laboratory tests and then followed their progress. We didn’t interfere with or treat the couples in any way. We just observed their progress and then analysed the data. This was the first time that a large study of its kind was undertaken and it revealed some interesting results.

Our study found a clear age-related decline in fertility in both males and females. At the time, this was very surprising as previously women had been allotted all the responsibility for the age-related decline in fertility. But we were able to demonstrate that both male and female ages were independently associated with each of miscarriage and infertility. Both men and women also had slightly enhanced fertility if they paired with a younger partner. Our results have since been supported by several other studies. There is no doubt that fertility declines with age in both men and women.

I am aware, like everyone else, of occasional remarkable stories of men fathering children when they are very old. Some of these stories are probably true. Others may be cases of mistaken paternity. Some women also conceive naturally after 50 but these cases are exceptional and far from normal.

The major decline in fertility is obvious shortly after age 35 and the change is quite dramatic. In women aged between 20 and 34 the infertility rate is about 10%. This doubles to over 21% between 35 and 39 and rises to over 30% at age 40 and increases dramatically from then age. The male age effect is very similar to the female.

Just as the rate of infertility increases with ageing, so sadly does the rate of miscarriage. I regularly receive emails or phone calls from women telling me that they had no problems conceiving when they were young but now, having decided to have another child, they are experiencing difficulty. Often the decision to have more children is associated with a new relationship. Sometimes the caller has not had any children previously and has met the love of her life when she was older.

The story shared by many couples who are experiencing age-related difficulties is similar to this. “We have been trying to conceive for some months without success and didn’t think anything was going to happen. Month after month we were disappointed. About eight months ago we were delighted to find ‘ourselves’ pregnant. We were so excited and joyful as we planned our new life with our new family. Then one morning devastation occurred and our doctor confirmed our tragedy. We had miscarried”. Most of the couples who contact me have had more than one miscarriage and some have had tests that have shown that the embryo had carried an extra chromosome.

In the PALS study, the rates of miscarriage were 10% at 25-29, 18% between ages 30 and 34, 23% between ages 35 and 39 and 70% in women 40 and older. These rates, are of course not absolute and women’s chronological age is not necessarily an exact match to her reproductive age but this is the general picture. It should be noted that we had very few women in the oldest age group and the average result could be slightly better than this. In general though the situation from about age 40 is grim.

Please visit my website to purchase a copy of “Starting your family after 30”, an e-book that gives all the facts and figures and the latest advice on how to increase your chances of success.

Dr Judy Ford is an internationally respected geneticist who has undertaken considerable research into the causes of miscarriage , causes of infertility and birth defects . Her research has shown that most problems are preventable through changes to healthy lifestyles and healthy habits. More information can be found on her website http://www.ez-fertility.co.uk .