But what do you do if, because of your genetic inheritance, you are at risk of conceiving twins when all you want or can afford is just one child? Even for many who defend a woman's right to choose, selective termination is a difficult, more complex issue, fraught with emotion and arguments about possible effects on the remaining foetus.
One couple who were at risk of having a twin pregnancy believe they have found an simple alternative to selective termination. For many people, it may be an option they would want to consider - especially as statistics show the incidence of twins is rising.
In 1994, 8,451 pairs of twins were born - one in 80 births, compared with one in just over 100 births 10 years before. The increase is partly due to infertility treatments, but also because more women are putting off pregnancy until a later date - and a woman's chances of having twins rises as she reaches her late thirties. Clearly, not everyone wants more than one baby. The number of selective reductions doubled between 1993 and 1994 to 73 a year in England and Wales.
Nicky Halfyard and her partner Quentin knew their risk of having twins was high. Nicky, 36, is a non-identical twin herself and her first (unplanned) pregnancy had been twins; she lost one of the babies early on. With one child already (Rosie), the Halfyards wanted to make sure they just had one baby next time round. This was partly doing their bit to control the population, but also because they wanted to have time and energy to devote to their children, something they felt would be difficult with three.
Given that doctors now enable couples to create multiple births, conceive in a test tube or control fertility by swallowing a pill, the Halfyards felt, not unreasonably, that someone might be able to help them. They were wrong.
"We spoke to our GP but she wasn't very sympathetic at all," says Nicky. "She basically said that you have got to leave it in the hands of God. She thought we were being unreasonable. But children should be wanted. We felt avoiding a twin pregnancy was as much a part of family planning as contraception. We had heard about fertility treatment, so we thought there must be some way of dealing with it."
After ringing round several London teaching hospitals for advice, the Halfyards eventually got some advice from King's College Hospital. But the choices were limited.
"Selective abortion was probably as good as it got," says Quentin. "If there are too many foetuses, they pick one at random and inject it with potassium cyanide. I wasn't considering anything like that and I didn't even suggest it to Nicky, as I knew she would reject it."
Another option was to fertilise the egg in a test tube, making sure only one embryo was replaced in Nicky's womb. This is a common procedure in fertility treatment and would have ensured only one baby.
But before they resorted to such an invasive procedure, the doctors at King's suggested scanning Nicky each month after ovulation to see how many eggs she had produced. As a non-identical twin herself, there was less chance that she would have identical twins from the same egg. The scan is a simple procedure, common in fertility treatment. In a month where she produced just one egg, the Halfyards could have unprotected sex. More than one, and out came the condoms.
The couple opted for the scan. After the first one, which revealed just one egg that month, they conceived straight away. Nine months later, their son, Gus, now one and a half, was born. They paid less than pounds 100 for the service - pounds 55 for the initial consultation, then pounds 20 on top for the scan, although couples having more than one scan would pay pounds 20 each time.
Quentin wonders why more people in the same boat haven't been advised to follow this course. "No one I spoke to had ever heard of this before," he says. "No one had bothered to think about it. At the moment, no one knows that the option exists."
So could a monthly scan be the answer for other couples at high risk of conceiving? Obviously, it would work only for non-identical twins, where two eggs are fertilised. But two-thirds of twins born in Britain fall into that category.
Dr Ruth Curson, the Halfyards' consultant at King's, who suggested the scan, doubts whether many other couples would feel so strongly as to go to such lengths.
"I don't feel a lot of people will feel they have got a very strong family history of twins or feel so concerned. But if they do, it is a technique that is possible. It is a luxury, but it is a very cheap luxury.
"It was incredibly simple. All you do is look to see how many follicles are developing on the ovaries. I can't see any ethical problem.
"You might say you are trying to organise your life too carefully and ought to leave it to chance, but Quentin was very concerned about the number of children they had. I would rather people thought it through beforehand and had a clear idea of whether they could cope with one or two children. I much prefer this as a solution than selective reduction."
Nicky Halfyard openly acknowledges that her husband felt more strongly about avoiding twins than she did - a difference in feeling that would have made selective reduction unthinkable. But she, too, felt happy with the solution, arguing that it is no different from family planning. "It was done before conception. I feel very strongly that children should be wanted," she says. Her husband is adamant that there is no ethical issue at all. "We are talking about an egg and a sperm that have not joined together."
Even Countess Quintavalle can find no objection to the method. "There is clearly no destruction of life in a procedure like this," she agrees. "There's a huge difference between selective reduction and avoiding conceiving twins."
But she is cautious about the idea. "I'm not saying it doesn't feel right; it seems just a further step to delivering goods you want. It is a long way from children being a gift. There is a sense here that a child is something you design."
Leaving aside the views of the anti-abortionists, is this method of avoiding twins taking the idea of designer pregnancy too far, or is it a sensible option which should be made more widely available? It is less distressing than selective termination for those who could not cope with the demands of caring for two babies at once.
Mary Lowe, of the Twins and Multiple Births Association, welcomes the initiative: "If someone is set against having twins, it seems a fairly non-invasive method," she says. "But there can be joys and rewards to having twins, even though there are difficulties and problems"n
The Twins and Multiple Births Association runs a helpline from 7pm to 11pm on weekdays and 10am to 11pm at weekends (01732 868000).Reuse content