In the opening scene of Bridget Jones' Diary, the lecherous Uncle Geoffrey sidles up to Bridget at her mother's turkey curry buffet and asks that dreaded question: when is she going to get "sprogged up"? "You career girls," he leers, "can't put it off forever. Tick-tock, tick-tock".
Childless women - whether they're 32, like Bridget, or 67, like Dame Helen Mirren, whose comments in this month's Vogue have reignited the debate on childlessness - have had to put up with jibes like this for centuries. When a woman reaches a certain age, she is expected to start thinking about having children. If she doesn't, society demands an answer. But why doesn't she have children, people will whisper. Isn't she able to have children? Doesn't she want children?
In the past, Mirren has answered with defiance. "I have no maternal instinct whatsoever," she once said. "Motherhood holds no interest for me." In her latest interview, however, Mirren, who has been married for 15 years, admits that she always expected to be a mother. "It was not my destiny," she says. "I kept thinking it would be, waiting for it to happen, but it never did, and I didn't care what people thought."
Women, she adds, never gave her a hard time: "it was only boring old men. And whenever they went 'What? No children? Well, you'd better get on with it, old girl,' I'd say 'No! F--- off!'"
In a society where not having children remains taboo, Mirren's honesty is refreshing. Childlessness is rarely talked about, nor the reasons addressed. Female celebrities without children (Kylie Minogue, Cameron Diaz and Jennifer Aniston among them) are subjected to are-they-aren't-they pregnancy stories. Women who reach their mid-thirties without giving birth are forced to justify their decision to family, friends - and often strangers. "The expectation is that they will marry and have children," explains Sue Fagalde Lick, author of Childless by Marriage. "If they don't, everyone wants to know what's wrong with them."
Mirren's words couldn't be timelier. Childlessness is on the rise - not just in Britain but worldwide. Latest estimates suggest that 25 per cent of women in Britain of childbearing age will never have a baby. The proportion of women without children has almost doubled since the Nineties, according to the Office for National Statistics, with one in five 45-year-olds yet to start a family. Overseas, one in five American women in their early forties is childless, rising to a third of women in the same age bracket in Germany and Japan.
Their reasons for not having children are varied, often deeply personal - and sometimes, like Mirren's, circumstantial. The author Lionel Shriver is one of a growing number of women who are childless by choice, also known as "child-free", or - as she puts it - an "Anti-Mom". In response to criticism of her 2005 novel, We Need to Talk About Kevin, which some labelled "hostile to family", Shriver wrote a series of articles justifying her decision not to have children. "I've had all the time in the world to have babies," she said. "I am married. I've been in perfect reproductive health. I could have afforded children, financially. I just didn't want them. They are untidy… they are ungrateful. They would have siphoned too much time away from the writing of my precious books."
Over the years, choosing not to have children has become increasingly acceptable. Centuries ago, failing to give birth to a male heir was grounds for divorce - Henry VIII left Catherine of Aragon, his wife of more than 20 years, for this reason. Childless monarchs, such as Elizabeth I and Queen Anne, were frowned upon, both losing their families the throne upon their death. Even in the last century, before contraception and infertility were properly understood, childlessness was blamed on the female, with wealthy families paying poorer ones to provide heirs.
When the Pill was made widely available in 1961, all this changed. About the same time women began to enter the workforce, parenthood was delayed until later in life, and having children - as the costs of education spiralled - became increasingly expensive. "In the old days, a woman was defined as a person who had babies," explains the author Fay Weldon. "But now, post-contraception and with the ability to earn and be considered equal, all women have turned out not to be the same."
An ONS study in 2010 found that just one in nine women born in 1938 remained childless, rising to one in five women born in 1965. It is projected that a quarter of 45-year-olds will be childless by 2018.
Some, like author Hilary Mantel and presenter Anthea Turner, are unable to have children. Author Jilly Cooper, who adopted a son, Felix, and a daughter, Emily, when she discovered she couldn't have children shortly after she got married, says she felt "hideous pressure" to give birth.
"At the time I was trying and trying, and I was just so desperate to have my own children," Cooper explains. "People kept saying to me 'Oh, you can be our godmother', but I wanted my own. I was lucky in adopting two absolutely heavenly babies. There is an appalling pressure on women to have children; it is terrible. There's always been that idea of: 'Oh you're just a mother, you're just a housewife'. But if you can't even be that, it's heartbreaking."
Nowadays, parenting is an industry, buoyed by mother-and-baby groups, self-help books and web forums such as Mumsnet, filled with opinionated parents debating the pros and cons of reusable nappies, baby iPads and stay-at-home dads. Parenthood has been elevated to the aspirational; a sort of exclusive, members-only club, which sees those who don't have children as selfish or individualistic.
"I don't think the pressure to have children comes from other mothers, so much as society at large," says Justine Roberts, co-founder of Mumsnet. "We have pretty strong prejudices about what women ought to do. Our users would be absolutely behind women who choose not to have children."
But Roberts admits that childless women need better support, with more advocates like Mirren speaking up to raise awareness of the stigma they feel. "Because we are one of the highest-profile voices for women, those who aren't mothers might understandably think: 'Hang on, that doesn't represent me at all. Who speaks for me?'" she adds.
Sue Lick agrees. "In my generation, motherhood was the default position," she says. "If you didn't have children, people wanted to know why, perhaps assuming you were infertile, selfish or whatever. But with today's young women, it's often a matter of timing and choices, and folks are starting to understand that."
Ultimately, whether a woman has, or does not have, children is a personal matter. Mirren speaks for a growing group of intelligent, professional - but largely invisible - women for whom having children isn't a part of their lives. And they're none the worse for it.
"You will find them among the higher educated, the ambitious and the high-flyers," explains Weldon. "They, like Mirren, have a different but equal service to make to the community. They should be praised, not condemned."