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.
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.