Catherine Bright's third pregnancy was different. During the first two, whenever the baby moved she placed a comforting hand on her abdomen to feel the growing infant.

Not this time. It wasn't a case of being uncaring. It was simply a way to keep a distance between herself and the twins growing inside her.

She needed that detachment because the twins were not hers. She was not the mother, nor was her husband, Kevin, the father. Days after they were born, the babies were handed over to another couple, the genetic parents.

Catherine, 28, and the couple for whom she gave birth, are among a small but growing number of people in Britain involved in surrogacy. Last week campaigners celebrated the 500th surrogate birth in the country.

To some, surrogacy is a risky business, a moral and ethical minefield, with few, if any, happy endings. For others it's a positive, fulfilling enterprise that brings joy to infertile couples who would otherwise be denied the children they long for.

Horror stories of money-grabbing surrogate mums refusing to hand over the baby, courtroom wrangles over custody, and commissioning mothers unable to bond with their new child, have contributed to surrogacy's bad image.

Cases such as that heard in the New Jersey Supreme Court in 1988 (where the surrogate mother sought custody of Baby M) served to fuel the moral panic.

But research by City University, London, shows that such cases are much the exception, at least in Britain. In almost all cases the arrangement works. Often relationships are so good that both mothers are at the birth - sometimes fathers, too.

The two mothers often go to prenatal appointments together. In only one in 10 cases does the relationship end once the baby is born.

In the biggest study yet of surrogacy in Britain, the City University researchers looked at 42 couples with a year-old child born through surrogacy. Research focused on the experiences of commissioning couples, and shed fascinating light on how surrogacy is arranged and the relationship with the birth mother.

More than half the couples had the traditional form of surrogacy, where the surrogate mother and the commissioning father are the genetic parents of the child, with conception through artificial insemination.

Sixteen couples had full surrogacy, a technique where the surrogate mother is implanted with an embryo created by the commissioning couple. In full surrogacy, the role of the surrogate mother is purely gestational and the child is genetically related to both commissioning parents.

The research shows that the typical age of the commissioning mother in Britain is 35, and that, on average, she and her partner have been trying for more than seven years to have a child.

The most common reason for opting for surrogacy was repeated IVF failures, experienced by 43 per cent of the women. The second most cited reason was that the woman had no uterus, sometimes as a result of an emergency hysterectomy.

Crucial to the success of surrogacy is the relationship between the commissioning couple and the surrogate. Of the 42 couples, 69 per cent had not known the surrogate mother, and all but one had met through the agency Childlessness Overcome Through Surrogacy (Cots); 17 per cent were relatives of the commissioning mother and 14 per cent were friends.

Some have argued that surrogacy with a stranger is potentially hazardous because of the levels of trust needed. Others have argued that opting for a relative or friend may put undue pressure on them. In Israel it is illegal for the surrogate mother to be a relative.

It is also clear that commissioning mothers seem to be more involved than fathers with the surrogate during the pregnancy, in that they see her more often.

"It is possible that sharing the pregnancy in this way can help the commissioning mother to feel connected to the unborn child and, in the case of partial surrogacy, to come to terms with the fact that she is not the genetic mother," say the researchers.

Of the relationships they saw, most were good, with little sign of conflict during the pregnancy and no evidence of serious friction. There was only one case of a surrogate mother having slight doubts about handing the child over. All but one of the commissioning mothers had no difficulty in accepting the baby.

Other research, however, has shown higher levels of concern. A Journal of Reproductive and Infant Psychology study of 19 surrogate mothers found that five were distressed about parting with the child.

A key factor is the future role of the surrogate mother. In the surrogacies where the mothers were strangers, two-thirds involved the surrogate mother having occasional contact with the child, and in one in 10 cases the couples wanted her to play a special role in the child's life by, for example, going to birthday parties. In one in 10 cases it was arranged that the surrogate would not be involved after the birth.

"There is little evidence in support of the theory that commissioning mothers may feel insecure about the surrogate mother's involvement with the child," researchers say. "Nearly all the commissioning mothers were positive about this and felt that their child would benefit from it.

"Despite the concerns that have been commonly voiced about surrogacy, the commissioning parents generally perceived the surrogacy arrangement as a positive experience and one that they would recommend."

Bright was drawn to become a surrogate mother by a desire to help others.

"I first thought about being a surrogate mother four years ago," she says. "With my first daughter I fell pregnant naturally but they then told me I had polycystic ovaries and I might need IVF treatment.

"That worried me a lot. Fortunately, I fell pregnant naturally with my second daughter [she has since had a third], but that fear spurred me to help other people and I got in touch with Cots.

"They did an assessment, pointed out the pros and cons, and explained everything. They then sent us details on three couples and we chose one - because of their bad luck with previous pregnancies.

"We spoke to them on the phone and then about six weeks after we chose them we met. We then decided to go ahead. "When it came to [the commissioning parents] taking the twins, I had no change of heart. I will see them on birthdays and at Christmas, and we keep in touch. We were paid expenses, but for me the satisfaction was helping someone to complete their family. I am working with another couple now - I want to do it four times."

Susan Rosenwasser and her husband, Oliver, had tried for 10 years to have a child before turning to surrogacy.

"I had miscarriages, failed IVF, failed egg donor, and surrogacy was the last resort," 48-year old Susan says. "About a year and a half ago we joined Cots, and Gill was the first surrogate who decided to take us. Gill conceived at home with my husband's sperm, and it was her sixth surrogate birth.

"Baby Alexandra was born six weeks ago. We paid £12,000 [$32,500]. But you can't put a price on a baby that you have been trying for 10 years to have."