Modern medicine is already allowing women to have children far later in life than their ancestors, but how far can female fertility really be extended?
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“It’s one of nature’s great inequities,” says Dagan Wells, professor of reproductive medicine at the University of Oxford. He is referring to the progressive, and largely irreversible, decline in female fertility from the age of 35 years onwards.

Men also experience a decline in their baby-making ability as they get older, but this fall in fertility tends to start later and occur much more slowly than in women. The fertility rate for men tends to begin falling around the age of 40-45 years old.

But when exactly does a woman’s fertility start declining? And when does that decline result in the end of natural fertility?

For millennia, women have been getting pregnant and bearing children in their teens and early 20s – not much different from the Krapina Neanderthals, living in Northern Croatia 30,000 years ago, whose fossilised remains suggest gave birth to their first child at 15 years of age. Prior to the 1960s, women in the US were having their first child on average at around the age of 21.

In 2017, however, the average age of mothers giving birth in all OECD countries was 30. Just under half (44%) of all live births in England and Wales in the same year were to mothers aged 30 while the average age of women giving birth to their first child in South Korea was 31.

Quality matters

But female fertility isn’t just about the quantity of eggs. Quality matters too, and is much more technically challenging to assess than egg numbers. While egg counts decline as women age, so does the quality of the chromosomes and the DNA contained within each egg.

The journey, not the destination

Mothers don’t just need to contend with their fertility as they get older, but also greater risks during pregnancy, labour and delivery. The First and Second Trimester Evaluation of Risk (Faster) trial, a US study funded by the US National Institute of Child Health and Human Development (NICHD), looked at the health records of over 36,000 women. They found mothers over 40 were two to three times more likely to experience health problems during pregnancy including diabetes and high blood pressure. They were twice as likely to experience bleeding from their placentas, have a caesarean delivery and to lose their baby later on in pregnancy.

The significant advances in reproductive medicine over the past decades have greatly increased the safety, success, accessibility and affordability of artificial reproductive techniques. Approximately 230 babies are born in the UK each year to women aged 50and over while 9% of all first-time mothers in the US were aged above 35 in 2014.

But as we have seen, these techniques are still limited to a degree by the age of the egg. This is, not least, because of the effects of ageing on the DNA, but also because older eggs have been exposed to environmental toxins for a longer amount of time. It is possible, of course, for women to undergo IVF using a donated egg from a younger woman. Nearly all fertility clinics across the world now also offer women an option to store their eggs, frozen in time, until she is ready for them to be thawed, fertilised and transplanted into her womb.

But while science is making commendable steps to help prolong the ticking clock of female fertility, it may not ever be possible to keep it going indefinitely. The decline of natural female fertility is as inevitable as it is universal.