Making Babies

Most women over the age of 25 will be familiar with those concerned looks from older relatives followed up by a whispered: "Isn't it about time you started thinking of having a baby?" But what if much of what we have been led to believe about the impact of age on fertility is not true?

Take this often-cited statistic: one out of three women over the age of 35 will not have conceived after a year of trying.

Jean Twenge, a psychologist at San Diego State University in the US, was 34, recently remarried, and looking to start a family, when she heard it from her doctor.

"That was very frightening to me, as it is to many women who are in their 30s," she says.

Confronted with those odds, she wanted to find out where the statistic had come from. And she discovered something quite amazing.

"The data on which that statistic is based is from 1700s France. They put together all these church birth records and then came up with these statistics about how likely it was [someone would] get pregnant after certain ages."

These are women who had no access to modern healthcare, nutrition or even electricity. Why would any researcher think they can tell us something useful about modern-day fertility?

Well, actually, they do have one big advantage. They weren't using effective methods of birth control.

"I can empathize slightly with the researchers in this area," says Twenge, who traced the source of the data as she researched her book, An Impatient Woman's Guide to Getting Pregnant. "It is difficult to draw conclusions about age and fertility from modern populations where birth control is widely used."

But there are some studies which have looked at modern couples, and these do paint a rather different picture.

The most widely cited is a paper by David Dunson published in 2004, which found that 82% of women aged between 35 and 39 fell pregnant within a year. That's significantly better than the two-thirds chance drawn from the 300-year-old birth records.  Read more: