On this episode of X10, we talk about why fertility declines with age and what we can do about it.
How come women can’t have children after a certain age, and what can we do about it? Actually, that brings up another question: is it true that the odds get worse each year, or is it more like an on/off switch? Stick around to find out.
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Let’s start by looking at why a woman can’t have children after a certain age. The common explanation — that women run out of eggs — doesn’t tell the entire story. Yes, women lose hundreds of ovarian follicles every month, and, eventually, the number gets so low that menstruation becomes irregular and then menopause sets in.
But that idea has gotten a bit of a challenge from recent research, which showed that rare stem cells in the ovaries of adult women can produce new eggs. Even if that doesn’t change the basic picture, it might point us towards treatments that could extend fertility.
Another thing to keep in mind is that fertility isn’t just about quantity. Quality also matters. For example, chromosomal abnormalities seem to be more common in the eggs of older women. This makes the woman more likely to conceive a child with an abnormal number of chromosomes. That might mean having a child with Down’s syndrome, but in the majority of cases, it will result in a miscarriage.
Changes in hormonal regulation also play a role in declining fertility. That could explain why some women experience hot flashes despite still having a regular menstrual cycle. Like so many things we’ve talked about, this turns out to be a complex picture with lots of interacting parts. So, even if we develop therapies to extend our lifespan, extended fertility might not come as part of the package. It might call for separate therapies.
Likewise, we might find ways to let women who are already likely to live past the “fertility deadline” stay fertile if they want to, but let’s look at this “fertility deadline” business a bit more closely. The idea that women face a “ticking clock” appeared in a Washington Post article in 1978. Since then, it’s become an accepted fact of life that women have to balance having a career against having a child. But why? That original article didn’t make its case with statistics but with an anecdote about the “Composite Woman”. Let’s have a look at some numbers and see what we can find.
Here’s a graph from a 2002 paper entitled “The variability of female reproductive ageing”. It shows the number of births by women of a given age at three different time periods. The most obvious thing is that the average maternal age has gone up, but the more interesting thing is that the shape of the curve isn’t dramatically different. It’s a pretty normal-looking curve with a drop at around age 40, but it doesn’t run into a hard wall.
The article describes the drop at 40-41 as the “mean age at which female fertility comes to an end”. It’s about 10 years before the mean onset of menopause. But behind that average, there’s a lot of variability. Menopause can happen any time between 40 and 60, so we’d expect to see the same variability in fertility.
So, how does a woman’s fertility change as she ages?
Psychologist Jean Twenge surveyed what we know about this in an Atlantic article in 2013. She found that some of the statistics about the topic come from a 2004 article, which used data from French birth records between 1670 and 1830. She wrote, “In other words, millions of women are being told when to get pregnant based on statistics from a time before electricity, antibiotics, or fertility treatment.”
Twenge tracked down several studies using more recent data, and they showed that women in their late 30s had only a slightly lower chance of conceiving within a year than women in their late 20s.
So, it turns out that there’s a lot of variability in reproductive aging, which is hardly surprising — this is biology, after all. There doesn’t seem to be much of a decline through the late 30s, but there’s a significant drop about 10 years before menopause. Actually, that doesn’t answer the really important question, which is: Why are we only talking about women?
Concerns about reproductive aging in men are often brushed aside, but there’s no justification for that. There’s good evidence that older men have more mutations in their sperm, which means their children could suffer from a range of genetic abnormalities. Genetic defects can also make conception harder. A 2003 study found that couples in which the man was over 45 took five times as long to get pregnant as when the man was under 30 regardless of the woman’s age.
A 2019 review reported that sperm motility and semen volume decline with age, and children born to fathers over 45 had poorer health and were more likely to need intensive care. Delayed fatherhood has also been linked with children having schizophrenia or autism. The authors wrote that “The disparity between the social interpretation of men’s and women’s fertility and role in reproduction speaks volumes regarding our assumptions about gender roles, reproduction and family planning.” I couldn’t agree more.
Research on longevity may help alleviate these problems for elderly people. Until then, if we insist on talking about a ticking clock, maybe it should equally be for women and men.
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