That ‘fertility cliff’? It may be more of a gentle slope

The study that identified 35 as a cut-off year was based on 17th-century church records. More modern evidence is available

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20th October 2016

Let’s talk about time. The slow killer, the destroyer of all hope, the pitiless blank-eyed enemy who, with each passing second, bears you further from all the joys you once knew and toward decrepitude and the void. Because it gets a bad rap.

I mean, obviously all the stuff I just said is true. Time is awful and it’s going to kill us all. But in one, highly specific, sense, time’s savagery is overstated. And that’s when it comes to having babies.

You’ll probably have read about how women who put off childbirth are gambling with their future. There are countless stories in newspapers of all stripes about women who concentrated on their careers in their twenties, then couldn’t have a baby in
their thirties. But there are about 9 million women of childbearing age in the UK, defined by the ONS as between 15 and 44. In a group that large, you’ll always find enough sad stories to fill a newspaper. What it doesn’t tell us is: what are your chances of getting pregnant at different ages?

Again, if you read the newspapers, you may have got the impression that there’s something called the ‘fertility cliff’ — you reach a certain age (often given as around 35) and your chances of having a baby drop off precipitously. And it’s obviously true that at some point women stop being able to have children. But it’s far from obvious that age 35 is a particularly relevant date. Where is this claim coming from?

As far as I can tell, it’s heavily based on a 2004 study published in the journal Human Reproduction, carried out by Henri Leridon. It looked at women reproducing under ‘natural conditions’ — i.e. no artificial birth control methods or fertility treatments — and found that 75 per cent of 30-year-olds will conceive within a year, compared with 66 per cent of 35-year-olds and 44 per cent of 40-year-olds. In four years of trying, it says, those numbers jump up to 91 per cent, 84 per cent and 64 per cent.

That feels like a cliff, doesn’t it? The change in the four-year figures looks particularly alarming between 35 and 40, going from about a better than five-out-of-six chance of conceiving to less than a two-in-three. You’re leaving one of the biggest decisions of your life to the roll of a dice, and hoping it doesn’t come up 1 or 2. But there’s a snag here. The data that these figures are based on comes from a very unexpected place. Or rather, a very unexpected time. Specifically, the
17th century.

Leridon and his colleagues looked at French church and civil records between 1670 and 1830, and compared how long it took for women to get pregnant, and how many children they had, after they got married. They found — unsurprisingly — that women who got married later had fewer children. And they found that the rate of childbirth dropped dramatically after the age of 33, to near-total ‘sterility’ at the
age of 45. It’s a clever way of getting hold of data, but it’s not clear it says much about modern life.

For one thing, it doesn’t take into account whether or not the women actually wanted any more children. Even though there weren’t artificial contraceptives around at the time, natural methods do exist. And, of course, older couples often just have sex less often. As far as I can tell from the study’s methods, it doesn’t make any effort to account for these things. And yet somehow this fertility cliff has become part of the modern conversation.

The good news is that there have been some — surprisingly few, but some — studies carried out with modern data. One was by David Dunson, and was also published in Human Reproduction in 2002. It looked at 780 modern European women, who, importantly, were trying to get pregnant, and measured the likelihood of getting pregnant each time they had sex. And it found a real drop-— women in their late twenties were twice as likely to get pregnant in any given sexual encounter as women in their late thirties. However, when you expand that out, as couples try over extended periods to get pregnant, the news was much less bleak.

Dunson himself put it like this: ‘Although we noted a decline in female fertility … what we found was a decrease in the probability of becoming pregnant per menstrual cycle, not in the probability of eventually achieving a pregnancy.’ About 82 per cent of couples aged 35 to 39 will still get pregnant within a year of trying, says Dunson – far better than the 66 per cent in the Leridon study. And that figure goes up to 90 per cent in two years.

The trouble is, it’s hard to get good data. Women who have problems with infertility anyway are the ones who are most likely to be still trying to have children in their late thirties and forties, which skews the figures and makes it look as though waiting is worse than it is. These are not the only studies, and there are still huge discussions over how steep the ‘cliff’ is and when it starts. But it seems that it’s much less bleak and frightening than popular opinion will tell you.

None of this is to say that there aren’t good reasons to have children earlier, if it fits with your life. Most obviously, if you do have fertility problems, it’s clearly better to find out sooner rather than later, so that you can do something about it. Also, your chances of meeting your grandkids and great-grandkids are obviously improved.

But there are good reasons to have them later as well; speaking as a new-ish dad, my eldest was born when I was 33 and I wasn’t mature enough to deal with it then. But at least I had a paying job, and a home, and a steady relationship. There is nothing irrational about putting off having children until you have a better chance of providing them with a stable life, even if the trade-off is a slightly reduced chance of pregnancy. And the evidence suggests that the trade-off isn’t as stark as you’ve probably been led to believe.


  • Lyndsey Harper

    I’m almost in my 30s, which is why I don’t want to delay the trying to conceive part. Even now that I’m not at the fertility cliff yet, I can honestly say it’s not easy. I’m taking conceiveeasy now to help boost my chances so I hope soon, I can see a positive result on my PT.

  • julian

    Might want to look at the study on the hitterites 1957 shows a clear fall in fertility after 35 years old.published in fertility and sterility