Women are born with between 2 million and 3 million eggs in their ovaries, says Child. "The common myth is that [you become infertile when] you run out of eggs," he says. "Occasionally that's the case, but generally it's that as you get older, the eggs that remain are of poorer quality. We don't really know why."
So you may continue ovulating, but your eggs are more likely to have genetic problems. That means that if they're fertilised, they're less likely to implant, and if they do implant, you're more likely to have a miscarriage further along in the pregnancy.
How late you want to leave it depends on lots of things. "We're living longer and working longer," says Munro. "We have different priorities – our priority isn't having a baby at 21, and that's a good thing. But our bodies haven't changed. We're still at peak fertility in our mid-twenties."
Adam Balen, a consultant in reproductive medicine and chair of the British Fertility Society, says that if you want a "really good" chance of having a larger family, it is better to start earlier. "If you just want one, you can probably wait until your early thirties and still have a really good chance," he tells BuzzFeed News. But if you want more than that, to give yourself the best shot, he says, it may be worth considering starting earlier.
But, Munro says, media focus on this can be unhelpful. "Women are inherently aware. We know this and don't need to be told," she says. "As clinicians, we'll work with the choices of the woman, so if she's busy in her career at age 35 and doesn't want an unplanned pregnancy, I'll give her options for good contraception."