Several years ago, Jessica Wintrip, a 43-year-old teacher from Taunton, was breastfeeding her one-year-old when she noticed her friend's three-month-old baby was not putting on weight. "So I just blurted out, 'Do you want me to feed him?' It was one of those things you say and you think, I've overstepped the mark." But her friend was grateful rather than shocked and agreed that she should try. "I fed him three times a day for three months - and she kept feeding him as well. It felt a real privilege to be able to do it. I knew how much my friend wanted to breastfeed, and if I hadn't stepped in, he would have had formula."

While in the developing world suckling someone else's baby is commonplace, here we see it as weird - but why? Formula milk has only been available since the early 1900s. Before then wet-nursing would have happened as a matter of course if the mother was ill or absent. In three generations it has become socially unacceptable. None the less, I know women in the UK who have fed each other's babies, although they acknowledge that they would not necessarily admit to it openly. I have also heard of several circles of parents where it is accepted that if you babysit for someone else's newborn, it is OK to breastfeed them (with the parents' consent). This informal wet-nursing is called "shared feeding" or "cross-nursing". And while this has always gone on on quietly in Britain, hiring someone else to breastfeed your child is becoming increasingly popular in Hollywood.

Cross-nursing also happens in extreme circumstances. Sarah (not her real name) was contacted by a local breastfeeding support group when a woman with a three-month-old baby was injured in a car crash. She was unconscious and unable to feed. The baby's father knew that the mother would ideally want to avoid feeding the baby formula so he contacted the organisation to ask for donations of breastmilk. "The first woman who turned up at the hospital to express milk said, 'This is a bit ridiculous. I might as well just feed the baby.' The father agreed."

In the end a group of five women wet-nursed the baby for a week until the mother recovered. This example is telling in that it shows that wet-nursing can be a pragmatic solution: it would have been very difficult for the family to feed this baby any other way (the father was also injured and hospitalised and in any case the baby had never taken a bottle or had any formula). It felt very intimate to feed another woman's child, says Sarah: "It was weird at first. But it was just a baby who needed milk and needed cuddling."

Wet-nursing is now making a comeback in China, after being banned for political reasons for decades. Rich Shanghai families are recruiting rural women as live-in nannies to feed their babies. Given that most well-heeled Chinese women do not work, these nannies-with-extras are more of a status symbol than a necessary accessory to a busy life. Banned under Mao as "decadent" in the post-war period, wet-nursing is a long-standing Chinese tradition: Pu Yi, the Last Emperor, was suckled into his teens. Now, China's nouveau riche are bringing it back, recruiting pregnant women, who leave their own child-ren at home with their grandparents (these babies then have to be wet-nursed themselves by another local woman). Their sacrifice is rewarded with a salary of up to five times the national average.

Perhaps even more intriguing than the actual trend itself is the way it has been reported in the west - with fascination disguised as disgust. One (male) reporter on a British broadsheet claimed with some excitement that these Chinese wet nurses are selected for their "superlative breasts". This, as any breastfeeding mother knows, is total nonsense - breasts of virtually any condition or size can produce milk. Babies do not give extra marks for beauty. The only breasts that have trouble producing milk (although it is not impossible) are fake ones. And, surprise, surprise, earlier this year a Los Angeles-based agency supplying wet nurses popped up. Certified Household Staffing claims it has on its books several Hollywood celebrities with breast implants who have requested lactating nannies. On its website, "wet nurse" is right next to valet, chauffeur and chef. Company director Robert Feinstock assured me over the phone that, yes, there was a demand, but declined to give any more details.

If feeding another woman's baby seems like the last taboo, it is one that exerts a fascination. (You can imagine what happens if you type the words "wet nurse" into Google: X-rated mammary heaven). In the film The Hand That Rocks the Cradle the moment we "know" that nanny Peyton (Rebecca de Mornay) is genuinely psychopathic is when she pulls her employer's crying baby out of the cot and puts it to her breast.

Perhaps the intimacy of shared feeding enhances the taboo. Rhonda Shaw, a sociologist at the Victoria University of Wellington, New Zealand, is author of a study entitled The Virtues of Cross-Nursing - and the Yuk Factor and is researching contemporary cross-nursing practices. We perceive feeding another woman's child to be somehow indecent, she says: "Adult meanings of eroticism get confused with breastfeeding as a sensual activity." She recently interviewed two single working mothers who have agreed to share childcare and cross-nurse each other's babies to fit around their work schedules. "It's always been practised. It just isn't reported," she says.

Extraordinarily, there have even been cases where there is an underlying suggestion that wet-nursing is borderline child abuse, says Shaw: "One state in the USA requires people to get a license before they provide someone else, other than their child, with breast milk. There was a case in Oklahoma in 2003 where a woman breastfed someone else's infant without consent, and faced a $500 [£257] fine and up to a year in jail on a 'morals' charge."

Gabrielle Palmer, lecturer in human nutrition at the London School of Hygiene and Tropical Medicine and author of The Politics of Breastfeeding, finds current attitudes perplexing. Once wet-nursing was so commonplace that Jane Austen mentions it in Emma: "For years it was a really good job for a woman. In 17th- and 18th-century Britain a woman would earn more money as a wet nurse than her husband could as a labourer. And if you were a royal wet nurse you would be honoured for life."

She has personal experience of the practice (her children were born in the 1970s): "I wouldn't mind another mother feeding my baby - if it was my sister or my best friend that would be fine. In the 1970s, you would get breastfeeding circles within the babysitting circle - you'd ask for somebody lactating. There is an idea that 'this is a special relationship' - it is that special bond - but I'd rather a baby had another woman's milk than formula."

Given the current climate, Palmer advises against anyone telling the authorities they are cross-nursing: "I know adoptive women who have breastfed their [adopted] children and the health authorities have been appalled. Far too many health professionals are hung up about breastfeeding."

The practice is officially frowned on in the UK because HIV can be transferred through breast milk, although, as Shaw puts it, "If a woman knew she was HIV positive I think she would be unlikely to cross nurse someone else's infant." (This is a major problem in countries where wet-nursing is culturally acceptable but talking about and testing for HIV is not; in many parts of Africa, for example.)

There is no medical reason why women should not lactate indefinitely or feed more than one child simultaneously (known as "tandem feeding" when a mother feeds two siblings of different ages: Palmer estimates that some women would thoeretically be able to feed up to five babies). And there are many historical examples of wet-nursing, says Shaw: Naomi, from the Old Testament, relactated to feed her grandchild so Ruth, the infant's mother, could go to war after her husband was killed. Both Moses and Mohammed were wet-nursed after they were rescued from the bulrushes. Dr Naomi Baumslag, author of Milk, Money and Madness, wrote a celebration of the legendary wet nurse Judith Waterford: "In 1831, on her 81st birthday, she could still produce breast milk. In her prime she unfailingly produced two quarts [four pints] of breast milk a day."

All our objections to wet-nursing, says Shaw, are cultural: "The exchange of body fluids between different women and children, and the exposure of intimate bodily parts make some people uncomfortable. The hidden subtext of these debates has to do with perceptions of moral decency. Cultures with breast fetishes tend to conflate the sexual and erotic breast with the functional and lactating breast."

I wonder if our attitudes towards the "freakishness" of wet-nursing betray what we really think about breastfeeding itself. We still don't completely accept it as what it is - natural, normal, instinctive. And no superlative breasts required.