"My biological clock is ticking" is a common expression.
It indicates that a woman recognises that she is getting older and that she can't indefinitely postpone having children.
Perhaps it is wisest, if possible, to use the years between 30 and 35 as at the time for completing the family instead of 35.
Couples marry later now for many reasons, including careers, finances, travelling and general business.
Unfortunately, although this seems perfectly reasonable, the decision often has unpleasant consequences.
Both men and women are less fertile as they get older.
The facts are clear and, at least for the moment, there is no simple solution.
For women, there is a phase that is referred to as 'oopause' which occurs about 8-10 years before menopause.
At oopause, eggs have a very low fertility and a high risk of any pregnancy miscarrying.
The PALS - Pregnancy and lifestyle study was performed between 1989 and 1993.
600 couples who were trying to conceive answered detailed questionnaires, underwent certain laboratory tests, and allowed us to track their progress.
We just watched their progress and did not intervene in any way.
This was the first time that a comprehensive study of this kind had been carried out and it had some surprising results.
There was a large age-related decline in fertility in both men and women.
At the time, it was very surprising because women were previously blamed for all of the age-related decrease in fertility.
But we were able to show conclusively that the ageing in both men and women increased the chances of miscarriage and infertility.
Both men and women showed slightly improved fertility when they were paired with a younger partner.
Everyone has heard remarkable stories about men fathering children when they are very old.
Most of these claims have not been tested but there is little doubt that at least some of the stories are true.
Some women have normal pregnancies after 50, but these cases are exceptions and far from usual.
The biggest decline in fertility occurs soon after 35 and the change is quite dramatic.
For women aged between 20 and 34, the rate of infertility is about 10%.
This more than doubles to 21% between 35 and 39 and is more than 30% at40.
The rate increases dramatically after 40 and the effect of male ageing is very similar.
At the same time as the rate of infertility increases with ageing of both men and women so unfortunately does the rate of miscarriages.
The history of most couples who have difficulties associated with age is the same.
"We have tried for several months without success and didn't think that we could conceive.
Month by month, we have been disappointed.
About eight months ago, we were pleased to discover that 'we' were pregnant.
Excitedly we planned our new life with our new family.
Then one morning, we were devastated to find that we had miscarried".
In the PALS study, the rate of miscarriage at ages 25-29 was 10%,it rose to18% between 30 and 34 years old, 23% between 35 and 39 years and then 70% for women, 40 years and older.
These values are not absolute and chronological age of the woman is not necessarily an exact match of her biological age, but this is the normal pattern.
In general there is only quite a low chance of a woman who is aged 40 or older having a normal conception without any problems.
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