Mel had her first baby two years ago. It was a textbook pregnancy, and today her son is a happy, healthy toddler. But for Mel, the baby's delivery changed everything. She is doubly incontinent; sex is difficult and painful; she rarely goes out socially, and she has only been able to return to work in the last two months.

Her baby's difficult delivery caused an obstetric fistula – a fissure, or hole, between her rectal and vaginal passages. She also suffered nerve damage and her brain no longer registers the signals when she needs to urinate or empty her bowels: in the long term she may need a colostomy.

This week, doctors from around the world gathered in Dakar for the annual conference of the International Society of Obstetric Fistula Surgeons. Their discussions centred on the problem of fistula in the developing world, where it is a common complication of childbirth. In the UK, the number of women affected is very small, but the effect on their lives is colossal.

Mel says that, were it not for the fact that her workplace is semi-medical, she probably would not have been able to return to her job. "I'm lucky because the people I work with understand incontinence, and that helps," she says. "But it's absolutely life-changing: I have to get up at 5.30am every day because I need to eat a meal and then do a colonic irrigation before I can leave the house. I can't eat all day or I'd have an accident; I wear pads, but inevitably there are times when they leak. It's debilitating and demeaning; it undermines your confidence, and it makes you wonder why you ended up in this situation."

Mel isn't alone: Vikki Dutton, who lives in Essex and has three children aged eight, five and one, had a fourth- degree tear when she gave birth to her first child. "For a few hours after the birth, I had one hole instead of two. I spent two hours in surgery, and left hospital with more than 100 stitches. It was incredibly, unbelievably painful – like getting a paper cut in the most intimate part of your body, only 1,000 times worse."

And there are more stories like these – lots more. Two years ago a poster called Cyee started a thread on "birth injuries" on Mumsnet while she was lying in bed after surgical repair for a fistula. More than 1,395 posts have since been added. Under the veil of anonymity that web-chat allows, women pour out the pain, and tragedies, that they have hitherto suffered in silence: some tell how they can't bear to be touched, let alone have sex, a year and more on from the birth. Yet more tell of how they wouldn't – couldn't – contemplate a second child, so terrible was the physical fallout from the first.

Some of the women posting feel mothers-to-be need more information about birth injuries and how to avoid them; others say it is unfair to scare pregnant women, because injuries of the severity they are suffering are very rare. It is hard to pin down statistics, because no one collates them centrally: but Michelle Thornton, consultant colorectal surgeon for NHS Lanarkshire, says third- and fourth-degree tears happen in 3% of vaginal deliveries in countries such as the UK.

Not all these, of course, cause permanent or long-term damage: but some do, and what the Mumsnet posters agree on is that suffering a birth injury is a peculiarly lonely experience: outside of cyberspace, it's rarely mentioned. "It's hardly the sort of thing you can discuss at an antenatal coffee morning," says Mel. "And in any case, I couldn't get out of the house to go to an antenatal coffee morning."

Many women suffer in silence – often for years. "I've met women in their 50s at hospital appointments who are now seeking help for problems relating to childbirth decades ago," reports Cyee on Mumsnet. "Some have lived with faecal incontinence all that time and done nothing because they thought they were freaks and because of the stigma."

Maureen Treadwell at the Birth Trauma Association agrees: each week, she says, her organisation hears from women who haven't known where to turn for advice before. "It's a totally hidden problem, and it affects women's lives in devastating ways," she says. "Many of the women who contact us have rectal as well as urinary incontinence, and they can't have sex . . . for some, their relationship totally breaks down as a result of it all. Women tell us it makes them feel dirty, it wrecks their work, their home and their social life. And it's a total taboo."

Since many birth injuries remain hidden, known only to the woman herself (Mel says that in the early months after her baby was born, she would sleep in the spare room rather than admit her incontinence to her husband, let alone tell a GP about it), it is impossible to know how common it is. Treadwell says there are now more clinics, and more services, for women with colorectal injuries, many of which emanate in childbirth. The Guardian recently reported that the NHS is increasing its specialist support services for women suffering from birth trauma following a surge in cases: and though birth trauma encompasses problems other than birth injuries, the two are often related. Guy Foster, a solicitor at Irwin Mitchell who specialises in obstetric medical negligence cases, says more women are contacting lawyers – although whether that is because more injuries are occurring, or there is just more awareness of them, is unclear.

Thornton is convinced the incidence is far higher than is generally thought. "One study found that between 25 and 40% of patients will have a birth injury of some kind if you actually look for it," she says. "It's much more widespread than anyone believes."

Her bugbear, she says, is that women aren't being told there are risks to vaginal delivery just as there are risks to caesarean delivery. "We know that certain circumstances make birth injury more likely," she says. "Big babies are a definite risk factor; a rapid second stage of labour is a risk factor. Having torn in a previous delivery is a major risk factor; augmentation is a risk factor, and so too is a forceps delivery."

The problem, as she points out, is that by the time you get to needing a forceps delivery, the baby is too low down to opt for a section. This, she says, is why so many obstetricians opt for caesareans when giving birth themselves.

Treadwell worries that the incidence of birth injuries is likely to rise. "Pregnant women are heavier than they used to be, which means that babies are heavier, and that means we're likely to see more birth injuries. What's more, pressure to keep the caesarean rate down means obstetricians might be less likely to intervene and suggest a section even when the baby is large, or when the labour isn't progressing." Both Mel and Vikki, along with many of the Mumsnet posters, believe that healthcare staff should have noticed that the delivery was looking difficult, and suggested a section. "In my case, the midwife was supervising a trainee next door, so she kept popping in and out. If she'd kept a closer eye on things, maybe she'd have noticed that delivering my 10lb 12oz baby wasn't going well, and suggested a section instead," says Vikki.

On medical advice, she had her later babies by caesarean – and Mel has been told that, if she has a second child, she will need a surgical delivery. "If only they'd suggested that first time round," she says. "I love my baby to bits – but never in a million years would I have imagined how his birth would leave me."