According to BPAS's research on 156,751 women having abortions between 2011 and 2013, 42 per cent of women in their forties hadn't used contraception, compared with 36 per cent in their early thirties and 34 per cent in their late twenties. The organisation, Britain's largest abortion provider, pointed out that more abortions were carried out for women over 40 than among teenagers.
"Over the past few years, we have seen much scaremongering about older women's fertility," says Ann Furedi, chief executive of BPAS. "From career women leaving it too late to older women banking on IVF to conceive, these stories lead many women to dramatically underestimate their own fertility later in life.
"Fertility does decline as you get older. But the drop is not as great as we are sometimes led to believe. For women who don't want to fall pregnant, the message is simple: use contraception until you have passed your menopause."
Cherie Blair was astonished to discover, at 45, that she was carrying her fourth child. "I thought: 'I can't be, I'm too old. It must be menopause,'?" she said. Actress Halle Berry last year revealed she was pregnant aged 46. "This has been the biggest surprise of my life, to tell you the truth. I thought I was kind of past the point where this could be a reality for me."
TV and radio presenter Gaby Roslin was similarly stunned, aged 41, to learn she was having a second child. "It was a surprise. I said to my obstetrician: 'But I'm so old!' He told me I was talking nonsense and that he had women of 46 on his books - and he's right. He said it's not an age thing, it's down to how healthy you are."
Before the introduction of reliable contraception, older mothers were common, with women giving birth to their last child when they were grandmothers. In the Twenties, the average age a woman had her last child was 42.
Today, 40-something mothers are more likely to be first-timers, and their numbers are rising once again. Office for National Statistics figures show that pregnancy rates for over-40s have more than doubled in the past 24 years, with 14 conceptions per 1,000 women aged 40-plus compared with six per 1,000 in 1990.
Today, many 20-something women are saddled with student debt. The average age to buy a first property is now 35, the age when women's fertility supposedly "goes over a cliff". One in three British men and one in five women aged between 20 and 34 still lives with their parents. No wonder the average age for a British woman to have her first child is 30, and 35 for university-educated women.
Among my own middle-class peer group, none of my close friends had her first child before 30. The vast majority were older than 35 and several were in their forties. I became pregnant at 35 and 37, with no problems and no regrets that I'd spent my carefree twenties focusing on friends and career. Being labelled an "elderly primigravida" in my birth notes only made me laugh.
But panic among young women has been increasing. The tipping point came in 2002, when US academic Sylvia Ann Hewlett published Baby Hunger, containing the unnerving statistic (that was misleading, since it only covered a tiny sample) that 42 per cent of career women had no children at the age of 40, and most deeply regretted it.
I frequently meet women in their mid-30s who fret about their fertility. "My boyfriend's 10 years younger than me and doesn't like me pressuring him to marry me, but I'm 36"; "I left my husband because he was unfaithful - I should have done it sooner but I didn't dare because I wanted to be a mum"; and "I'm not sure I'm really in love with him, but I'm 34 so I'd better marry him than risk never being a mother," are just three stories I've heard in recent months.
"I was consumed by anxiety that my age meant doom," wrote the US academic Jean Twenge, whose recent article on fertility scaremongering in the Atlantic magazine went viral.
"I was not alone. Women on internet message boards write of scaling back their careers, or having fewer children than they'd like to, because they can't bear the thought of trying to get pregnant after 35." Twenge had three children, all born after her 35th birthday.
The fertility expert Zita West says that she constantly sees clients "panicking unnecessarily". "Modern life puts up so many hurdles for women in their twenties that it's not easy for them to have babies at the 'ideal' time, and then there's so much anxiety and impatience from clients in their thirties.
"Couples put huge pressure on one another during ovulation and it's increasingly common in my consultations to see men who have performance anxiety around sex and ovulation. They say: 'Oh, my God, it's never going to happen' when they've only been trying for three months, or they live in different countries and only have sex once a month. Often they rush into having IVF when they don't need it."
In fact, the true statistics about female fertility are far less terrifying than is widely believed. Women do lose 90 per cent of their eggs by 30, but that still leaves them with 10,000, when only one is needed to make a baby.
Then there's the statistic that one in three women aged between 35 and 39 will not be pregnant after a year of trying, taken from a 2004 article in the journal Human Reproduction. These figures do not come from large, scientifically conducted studies of contemporary women, but from French birth records from 1670 to 1830, covering women with no access to modern health care or nutrition.
In contrast, the few studies of women born in the 20th century and trying to conceive are markedly more positive. One 2004 study of 770 European women found that 82 per cent of 35- to 39-year-olds would conceive within a year if they had sex once a week, compared with a very similar 86 per cent of 27- to 34-year-olds.
Consultant gynaecologist Tina Cotzias agrees that "older" women shouldn't be daunted. "Yes, chances of pregnancy decline with age but this doesn't mean it will never happen to you as an individual. And, of course, there are many reasons why it might not be right for a woman to have babies in her twenties, not least that she may not have met the right man. What's important is not to scare single women, but to communicate to a 28-year-old who is with the man she wants to spend the rest of her life with that she might be better off trying for a baby now than delaying it 10 years."
Cotzias warns that statistics are gloomier for IVF patients, with only 4 per cent of IVF cycles ending in a live birth in women aged 42 and older. Miscarriage rates soar in the over-40s, from an average 7 per cent to 18 per cent, and the risk of stillbirth doubles.
On the plus side, research indicates that "older" mothers usually have more solid marriages, command higher salaries and live longer than women who have their children in their twenties. When interviewed, these women almost invariably report that choosing to delay motherhood was the best choice they've made.
Ellie Stoneley, from Cambridge, author of the blog Mush-Brained Ramblings, was 47 when she conceived her daughter - Hope, now two - after IVF. "I had a straightforward pregnancy, and the medical staff could not have been more supportive. I get tired from time to time, but so do all new parents and I know if I'd had Hope younger, I'd have been trying to go out more in the evenings and would have found it much harder to get up in the night. Now all my energy's focused on my daughter. I wish I'd had a child younger as I'd have loved lots, but I do feel incredibly blessed."
Ellen Arnison went on, aged 42, to give birth to a healthy son, now four.
"Being pregnant in my forties was tougher than in my thirties," says Arnison, author of another blog, In A Bun Dance. "But motherhood was easier. I was much more relaxed, because I was more confident.
"I could be paranoid, but I do sometimes feel there is a sexist agenda in telling women they must have babies at what's also a crucial time in their careers," she continues. "The truth is there's no 'best' time for a baby - you take what life brings you."
