Michelle and Simon Beardsell have a four-year-old son, Jack. They are now desperate to provide Jack with a brother or sister, but their desire has provoked some surprisingly negative attitudes. "I've been told I should be grateful and count my blessings that I have Jack," says Michelle, 33, from Gloucester. "I have even been told by a member of my family that I shouldn't follow a dream that could wreck my marriage."

This "dream" of a second child is controversial because the Beardsells conceived Jack via fertility treatment. It took three years - and two attempts at ICSI, where a single sperm is injected into the mother's egg - for Michelle to give birth. Now they must undergo treatment once again.

"When we had Jack, we felt lucky to finally have one," says Michelle. "But as time went by, we felt he gave us so much pleasure, why shouldn't we try for another? I always wanted two children, and Jack often asks why he doesn't have a baby brother or sister. No one asks people who have had children naturally why they want more, and I don't see why we should be any different. We paid for Jack, it wasn't just luck."

Recent guidelines from the National Institute for Clinical Excellence made a number of recommendations about how IVF should be provided. They included the offer of three cycles of IVF to couples where the woman was aged between 23 and 39. The NHS responded by saying it would offer these women one full cycle of treatment. However, it added that priority should be given to those couples that did not already have a child living with them.

"We had loads of sperm frozen when we started on fertility treatment, so it made sense for us to use them and try again," says Michelle. "But some people seem to think you have to justify yourself. They think you should be happy with an only child."

"Having another child through IVF is not something that's usually talked about," adds Angela Bish, who has just embarked on treatment for a second baby. She and her husband Simon had Molly, three, through IVF.

"I just think that Molly would like a brother or sister," says Bish, 35. "We are very happy to have her, but we don't really want an only child."

Rajat Goswamy is the medical director of the Harley Street Fertility Centre. Half of his patients are people who have had children before. And many of them say they are doing it for the sake of the first one.

"A lot of them simply don't want an only child," he says. "It's a personal decision, but generally, they come to me not just because they want to increase their family, but because they want the first child to have a sibling. If someone has a baby through us and they're under 40, I'd put money on the fact that they'll be back in two years for a second one."

Angela Bish, who is just starting on further treatment, says that having Molly was a very emotional experience. "We tried for seven years and were ecstatic when we had Molly. We've got embryos stored and we have to use them this year or next. I know that if we don't go for it, I'll always wonder what could have been."

But Goswamy thinks that the emotional pressures of trying for a second baby can be worse than going for a first. "When they are trying for a first child, if it doesn't work out, they accept it. But when it comes to a second one, if it doesn't work they feel that they are letting the first one down."

The Beardsells have had four attempts at further treatment since Jack was born. "We are absolutely devastated it hasn't worked," says Michelle. "It's been traumatic and you do think, why us? We have good-quality eggs and I know I can get pregnant and carry a baby."

They are now considering egg sharing. Their attempts at fertility treatment have so far cost them around £24,000, and egg sharing is not only much cheaper, but also a way for Michelle to help people who have not been able to even have one child. The couple now only have enough sperm left for three more attempts. They are tired and losing faith.

"I'm almost finished with it," says Michelle. "I feel exhausted and drained. The second time it really is harder. People ask when you're going to have another baby; you know you can do it and you know how much joy a child can bring. Then, when it doesn't work, you feel completely alone. You are like the odd one out and no one understands."