Women’s fertility is more complicated than you might have heard

Another risk is miscarriage. “The other thing that that happens in the late 30s is that the chromosomes in the X (chromosome) are getting more unstable – that’s why there’s an increased risk of chromosomal aberrations, such as Down syndrome. So, many of the pregnancies will end up in a miscarriage,” says Pinborg.

One very large study of more than 1.2 million pregnancies, for example, found that the risk of miscarriage was around 10% for women aged 20-24, but started to rise more steeply close to the age of 35, when it was over 20%. By age 42, more than half of intended pregnancies – nearly 55% – were lost.

Birth defects and stillbirth also become more common as we age, but generally closer to 40, not 35. One study of 1.2 million births registered in Norway from 1967 to 1998 looked at the average age of the parents, for example, and found that birth defects became markedly more common when the parents’ mean age was 40-44 (with a mean age of 38 for the mother and 45 for the father), while infant mortality rose around 35-39 (with a mean age of 34.5 for the mother and almost 39 for the father). “While the 40–44 parental age category had an increased risk relative to the benchmark group, it was vastly exceeded by the risk for the 45–49 parental age category,” the researchers write.

But younger wasn’t always better: couples with an average age of between 20 and 24, for example (where the mean age of the mother was 21), had the same infant mortality risk as those aged 40-44 (with the mother’s mean age of 38).

Even age-associated risks, however, are complicated. Egg quality is also impacted by factors like smoking, alcohol consumption, and obesity, for example.

Other risks commonly associated with “advanced maternal age” are nuanced, too. It’s common, for example, to hear that the risk of pre-eclampsia is higher after age 40. But one authoritative study which looked at more than 25,000 pregnancies over 10 years found that, after taking other risk factors into account, like whether mothers smoked, age alone did not increase the risk of pre-eclampsia.

The same study also found that other commonly discussed risks, such as that of an emergency caesarean-section or placenta previa, increased from the age of 40 or 45 – not 35. One exception was the risk of gestational diabetes, which increased from age 30 onwards.

Men matter, too

Meanwhile, the focus on female fertility alone can distract from the fact that male age matters, too. The study of European couples found that while a father’s age had no bearing on the likelihood of conceiving if he was 35 or under, that changed in the late 30s. “Among 35-year-old women, the proportion of couples failing to conceive within 12 cycles increases from 18% if the male partner is 35 years old to 28% if the male partner is 40,” the researchers write. After two years, that fell to 9% and 16%, respectively.