Age Matters

What is the optimal age to get pregnant? With the average age of New Zealand women having a child now at an all-time high of 30, it seems obvious that with the age of first parenthood increasing, women's fertility is decreasing. Fertility specialist Dr Richard Fisher looks at the research surrounding how age impacts on fertility, for both men and women.

Social change is always fascinating. Changes in the way we behave often lead to unintended consequences. One of the most significant adverse consequences arising from the change to older parenthood (long held to be consequentially "good for society") is that the incidence of subfertility is increasing for many couples. For many, conception is easy; but for an increasing number, it is becoming more difficult. This is not because there is a fundamental change in human ability to reproduce, it is simply a side effect of beginning one's reproductive life later than in the past.

The average age of first birth in New Zealand has climbed steadily since the 1970s. The situation now in 2008 is that it has reached 28. The average age of all New Zealand women giving birth is around 30 years. Clearly this is only an average, but what it does mean is that a significant number of people are trying to have their first baby later than they used to.

Human fecundity (the chance of conception per month) changes quite dramatically over time, with good evidence that it begins to decline in the late 20s and it falls more rapidly from the mid-30s. At age 30, the average chance of conception per month is around 20%, and by age 35, it has fallen to around 17% or 18%. By 40 it is down to 10% per month. At 43, most women have only around a 4% to 5% chance of conceiving each month.

Fecundity is a biological variable, and involves two people rather than one, but the primary determinant is usually the woman's age. That some women do conceive at 43 is certainly true, but also means that a significant number will not conceive, through no fault of their own, and with no underlying cause other than chance.

As women age, the likelihood of the egg that is released each month being normal decreases, and by age 40, around 85% to 90% of eggs are chromosomally abnormal.

At a time when couples would most like to conceive quickly, nature determines that conception will, more likely than not, occur slowly. To compound this, the incidence of miscarriage and later fetal loss increases as age increases. The risk of miscarriage increases from around 10-15% at age 30 to 35-40% at 40.

Article content written and reproduced from OHbaby!
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