The chance of conception per month is called fecundability, and understanding fecundability is key to understanding the effect of a woman's age on having a baby. Fecundity represents the chance of having a baby each month.
The difference between the two is the chance of having a miscarriage following conception. As age increases, the gap between these two date points increases and so does the rate of miscarriages.
Young women without fertility problems have about 25 per cent chance of having a baby if they try at the right time of the month. This falls to about 12 per cent at age 37 and 5 per cent at 42.
Given enough time, many older women can have that baby - even if it takes an average of eight months trying at 37; and 18 months at 42.
Unfortunately there is no magic way of measuring a particular woman's fecundability. At 37, how can you know whether you are Ms Average with a 12 per cent chance, Ms Higher-than-average with a chance of 18 per cent, or Ms Lower-than-average-but-still-normal with a chance of 6 per cent? You can't - all you can do is give yourself a reasonable amount of time to conceive and then seek advice if you do not become pregnant.
The problem is that the longer you wait the higher the chance you might conceive. But as time passes fecundability will reduce and if active treatment is required the chance of success is also diminished. The best advice we can give is to understand that time is important and to present early for assessment of potential adverse fertility factors and to plan rationally at what time intervention might be appropriate, if conception doesn't occur. To do this, see a fertility specialist early on in case you have any discoverable fertility factors.
Another factor is that advancing maternal age increases the risk of miscarriage from around 10-15 per cent at age 30 to 35-40 per cent at 40.
Paternal age may become important once a man's age increases beyond 45. Given that men tend to be older than their partners the combined effect may be very significant. Paternal age alone is of limited significance when it comes to conception.
For more than 20 years I have been talking about the effect of maternal age on fertility, however this information is still not widely understood. Most people are brought up to think about "when" they will have children, rather than "if", and hold firmly to the belief that such an "if" could not happen to them.
As always, there are competing messages in the media which make an assessment of likelihood difficult. The unexpected conception by women in their mid-40s becomes, quite rightly, news to be celebrated, but is usually expressed as the norm rather than the extraordinary.
A consequence of this delay in attempting to conceive is that more people are presenting to infertility clinics for assistance. Often there is no definable cause other than age, and, with luck, time alone will allow conception to occur. A normal 37-year-old will take seven to eight months to conceive, and the average 40-year-old will take up to 15 months.
Since 1990, there has been a four-fold increase in the use of IVF in women over 40. Around 20 per cent of all couples having IVF at Fertility Associates Auckland are now over the age of 40. IVF is a highly successful treatment in younger women and a relatively effective one in older women, but even with IVF, the chance of success depends on the age of the woman providing the eggs, and at 42 will often have only a 15 per cent chance of having a baby per attempt.
It has been widely stated (without any factual basis) that the reasons for delaying conception are primarily around women deciding to become educationally, professionally and financially more secure, quite apart from the independence that the emancipation of women has brought.
Two recent surveys, however, in Australia and New Zealand, both of infertile couples, strongly suggest that the lack of a suitable partner might well be the prime determinant. Whether women are getting more fussy, men are becoming less ready to commit, or whether there are just are not enough suitable men to go around, it is a fertile topic for further research.
The news is not all bad though. Knowledge and technology have moved swiftly over the last five years giving rise to investigations that help predict the difficulties that may arise.
Technology, however, will not overcome the biological imperative of age, and we cannot expect it to resolve what is essentially a social problem. Social change, with an understanding that if the opportunity is available, then attempting to conceive early rather than late, is likely to be the single biggest influence on changing the incidence of subfertility in our community.
Start the conversation early with your partner, with your GP or your specialist.
Dr Richard Fisher is co-founder of Fertility Associates
