Topic: How far can female fertility be extended?
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?
“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 what does this mean in the context of the ticking clock of female fertility?
For decades, scientists have associated the decline in female fertility with the age-related decrease in the number of eggs contained within a woman’s ovaries. Each, if fertilised, has the potential to grow into a baby.
Unlike men, whose reproductive organs produce millions of fresh sperm on a daily basis, women are born with all the eggs that they will ever possess. Moreover, this number steadily declines as a woman ages: from one million eggs at birth to 300,000 by puberty, 25,000 by the age of 37 and 1,000 by the age of 51. Of all these, however, just 300 to 400 eggs with baby-making-potential – normally just one a month – will mature and eventually be released from a woman’s ovaries through ovulation across her entire life. For reasons not yet fully understood, the rest undergo a natural process of degeneration and will never be ovulated.
Topic Discussed: How far can female fertility be extended?