Did you always picture yourself with a big family? You know, 2.5 kids, a dog, a back yard? Well now Dutch scientists have calculated at exactly what age you need to start trying to get pregnant to have the best chance of realising your dream.
The team pooled data from more than 58,000 women to figure out just how much fertility declines with age.
And the results are more than a little scary. If you want a big family but are waiting for the "right time", chances are you have been waiting too long. The team found that to have a 90 per cent chance of having three children without the use of IVF a woman should start trying to conceive at the age of 23. But the good news is that if you only want one child, you can wait until 32.
The average age of first-time mothers in Australia has been increasing, and is currently 28. The average age of Australian women who give birth in any one year is now 30, compared to 27.9 in 1991.
One of the creators of the new model, Dik Habbema from Erasmus University in Rotterdam in the Netherlands, told New Scientist magazine the team was trying to "fill a missing link in the decision-making process" for couples trying to decide what time was right.
"My son is 35 and many of his friends have a problem deciding when to have children because there are so many things they want to do," he said.
The data used to calculate the model goes back more than 300 years, but New Scientist notes that despite recent criticism about just how accurate research into the way fertility declines with age is, it's the only information we have about population-wide fertility in large groups that are not using contraception.
One professor thinks the age-chart produced by the research team is so important it should be given to every clinic, and perhaps even high schools.
"We should also be aiming this at sixth formers [college students] and university students, so that they're aware of how to plan their life," says Allan Pacey, professor of andrology at the University of Sheffield in the UK.
Mark Bowman, the president of the Fertility Society of Australia and medical director of Genea, said the research could be useful for couples who wanted to have children, but were unsure when to start.
"I think the general concept of saying to people, 'Look, if you want to have a family with two or three children you just can't assume that your general health or technology is going to do that if you leave it to your late 30s or early 40s to have a family' is important," he says. "We get people coming in with very unreal expectations".
But he said people shouldn't get too hung up on the numbers when social factors pushed people to meet partners later and start families later.
"We need better parental leave, better workplaces, and ideally better relationships too," he said. "But if you are in the right relationship space, you should just get on with it."
Associate Professor Bowman said too many people were confusing good health and fitness at older ages with fertility.
"The longevity of humans now is into the 80s, but we are still left with the same evolutionary issues that the human species has always had, and particularly for women, most women, fertility is really tough by the late 30s, and you will run out of good eggs in your late 40s.
"We are still stuck with the same issues we had 50,000 to 100,000 years ago, it's just that we live longer now".
The research team also used recent international data to calculate what age a woman should start trying to conceive if she was willing to also attempt IVF if things didn't work out.
But Associate Professor Bowman said it was wrong to think of IVF as a choice.
"IVF is a medical treatment, not a choice," he said. "You don't decide that because you are older IVF must be better than sex, that's the message I'm trying to get out to people. If you are older and you are destined to get pregnant on your own, it will probably happen in a few months or so, you don't need to jump to IVF".
While older men are more likely to have children with conditions including schizophrenia and Down Syndrome, their fertility does not appear to decline as strongly until they reach their late 40s.
