Amber Hartnell did not intend to have an orgasmic birth - it just happened. "Trying to have an orgasmic birth defeats the object," she says, "I just got into this ecstatic state where I had these peaks of orgasm. There were these rolling waves coming through me where I was laughing and crying. I didn't feel like I was having contractions. They were more like rushes. I did not actually experience pain, I experienced intense sensations."

For the majority of women who have given birth - and most of the men who have watched them - these "sensations" are a euphemism for pain. And they certainly don't feel anything like an orgasm. But Hartnell says this is definitely what happened during labour with her son Orus, now three and a half. She had a water birth at home over a 12-hour period and the "orgasmic rushes" kept coming from about two thirds of the way through. "It was the most overwhelming pleasure I have ever felt in my life," Hartnell says. "It was like an energetic movement through me." She did not do much to prepare for the birth, she says. "I do yoga to keep my body open and flexible and I have practised different forms of meditation. But I had only read one book about birth - I didn't want to fill my head with information, I just wanted to be open to the process."

Now her experience has gone global. Hartnell, 29, a full-time mother from the island of Kauai, Hawaii, features in a documentary, Orgasmic Birth, which aired on ABC's 20/20 in the US earlier this year and is being screened at special events all over the world, including several recent midwifery gatherings in the UK. Hartnell's husband had filmed her giving birth and they agreed to let the director use the footage. "It was a really major deal sharing the most intimate and vulnerable moments of my life with the world," she says. "But I had the sense that sharing this story could help other women to break through their fear and have a beautiful experience of birthing."

The response to the film has been one of both fascination and horror. For many women the idea that childbirth can be orgasmic is at best hippyish and possibly offensive - a notion that at once piles pressure on to women to find pleasure in giving birth, and seems to deny the pain the vast majority experience. One heavily pregnant blogger writes that she "can understand pain being natural in childbirth and letting your body take over and making it as enjoyable as you can. But orgasmic? No. Whoever finds that orgasmic needs help, in my opinion." Another suggests that they "wouldn't like to think my mother had an orgasm while giving birth to me." Hartnell, however, reports receiving hundreds of messages from enthusiastic supporters, including from several pregnant women who changed their birth plans after they had watched the film.

The title of the documentary is actually slightly misleading. Because while it features interviews with several women who swear they have had an orgasm during labour (and even shows eye-popping footage of them experiencing this), it is really about "undisturbed birth" - natural labour in a home setting, without drugs, or even gas and air. It is only in this environment that a handful of women are able to reach orgasm, says Marsden Wagner, MD, former director of women's and children's health for the World Health Organisation. "It's got to be how it is when you make love with someone," he says in the documentary, "It's got to be safe, secure and uninterrupted."

The documentary features Ina May Gaskin, the most famous birth activist in the US, whose book, Spiritual Midwifery, has been a birthing bible for celebrity earth mothers such as Davina McCall and the actor Isla Fisher. An advocate of "ecstatic birth" - pleasurable, drug-free labour - May Gaskin once conducted a poll of 151 women in which 32 reported having at least one orgasmic birth. She says that an ecstatic birth "is the best natural high that I know of. Women don't have a way to know how their body works until they really try it out in birth."

The idea that birth can be orgasmic isn't new. The British birth guru Sheila Kitzinger says that she has met "hundreds" of women during the course of her career who report experiencing orgasm during labour - some were hoping for it, others were taken completely by surprise. She herself has experienced it during three of her four labours (she has five daughters: one birth was twins). "It is difficult for a man to understand," she says "hard, too, for any woman who has had an average hospital birth. But it can be one of the most profound psychosexual experiences in a woman's life. Each contraction may bring a rush of joy so overwhelming that the pain recedes into the background." She puts this partly down to simple biology. "The pressure of the baby's head against the walls of the vagina and the fanning out of the tissues as the head descends bring for some women an unexpected sensation of sexual arousal, even of ecstasy." But is this really an orgasm? Or just a very unusual sensation? "It can be orgasmic. People recognise it as an orgasm. And it can be a multiple orgasm, one with each push."

The film's producer, birth educator Debra Pascali-Bonaro, says a woman's ability to feel intense physical pleasure during childbirth is "the best-kept secret". So well-kept that many women would argue that the phenomenon does not exist. There is debate over whether these women have really been experiencing a sexual climax, or are simply having some form of sado-masochistic response, mistaking intense pain for pleasure. After some critics in the US wrote the idea off as a "fairytale", one of the film's orgasmic subjects, Tamra Larter, a mother of two from New Jersey, clarified that she "felt something resembling an orgasm" and that the sensations she experienced "were something different than sex, but similar enough I feel OK using the word orgasmic."

Whether you believe in orgasmic birth or not, labour is not so unlike sex, says Christine Grabowska, senior lecturer in midwifery at Thames Valley University. "Labour does naturally involve a crescendo and those observing can hear that the noises women make are similar to those of love-making - which can embarrass their partners. French birthing guru Michel Odent has aligned the two and said that women should birth in the place they make love." She has known of women who have been able to "go with the rhythmic flow" and describe the sensation as a "huge orgasm".

Women in the documentary have been criticised online for kissing their husbands too much during labour, which some viewers find distasteful. Hartnell stresses that "there was never any sexual stimulation of any sort" during her labour. But many of the processes associated with labour also happen during sex, such as the release of oxytocin and endorphins into the bloodstream. Anyone who has had an overdue pregnancy knows that midwives recommend nipple stimulation and/or sex to bring on contractions.

Why then do we feel so uncomfortable about the idea of women having an orgasm when they are actually giving birth? "It crosses the margin of decency - which I think is wrong," says Kitzinger, "We're told that sex is different from childbirth. In the same way, it is considered indecent to experience intense physical satisfaction from breastfeeding."

Many women naturally resent the idea that they are supposed to "enjoy" labour and "embrace the pain", and even Kitzinger, who has been preaching the gospel of orgasmic birth for the past 30 years, agrees that it would be wrong to hold up orgasm as a new gold standard for women in labour. "The orgasm is a side effect not the goal," she says. "We don't want mothers or midwives feeling that they've failed if the woman doesn't have an orgasm. And I would like to say to all the men out there: please don't expect your partner to produce an orgasm. She may be faking it in bed but don't expect her to fake it during childbirth".