When does life begin? And when should the rights of the child, born or unborn, supersede those of his or her parents?

When does life begin? And when should the rights of the child, born or unborn, supersede those of his or her parents? For many, the concept of abortion is either morally wrong or a fundamental feminist right. But a recent article in a medical journal presents an interesting and rather unsettling dilemma stemming from the current law on abortion - and from the way in which the rights of the child are so different once it is outside the womb.

Writing in the Journal of Medical Ethics, Dr Robert Boyle, a paediatric registrar in London, describes the real-life case of a 31-year-old woman who was 23 weeks into her pregnancy when her waters broke. Doctors told her that, while they could take action to delay labour, the chances of her giving birth at 26 weeks were high.

If born at 23 weeks, there was only a 10 per chance of her baby surviving, and a one-in-two chance that it would suffer from some degree of disability. At 25 weeks, babies have a 50 per chance of survival and a significantly smaller risk of disability, although there are no complete figures for the likelihood of handicap.

The medical team treating the woman then told her and her husband that if the baby was born at 23 weeks and was in very poor condition, they and their doctors could make a joint decision not to resuscitate it. However, if the baby was born at 25 weeks, even with a disability, current guidelines would mean that the medical team would feel obliged to revive and actively treat the child, even if this was against the parents' wishes. This is because medical advances mean that at 25 weeks, premature babies are now considered "viable" human beings who should be treated as any adult patient would be.

The couple were plunged into a personal and ethical dilemma of the worst kind. The woman had seen the toll that caring for a disabled child takes on families, through her work as a teacher and as the aunt of a handicapped little girl. Both she and her husband were disturbed at the idea that, if their child was born at 25 weeks, they might have no say over what happened to it. They already had one child, and were inevitably worried about the impact that a possibly severely disabled sibling could have on their family. Faced with the statistics, and this added nightmare, they decided to abort their unborn child - which at 23 weeks, they had the right to do.

As Dr Boyle says: "If the couple in the case described were assured they would have the option of not resuscitating a child born at 25 weeks, then they might have continued with this pregnancy." Up until the 24th week, women can have an abortion on the grounds that it will safeguard their physical and mental health. But at 25 weeks, the law changes abruptly in favour of the child. Previous cases taken to the High Court have upheld the medical guidelines. In one of the best known and often-quoted rulings, the parents of a girl born with Down syndrome who had refused to consent to life-saving surgery for their child were overruled by the Court of Appeal. The parents argued that their child's quality of life was so awful that she should be allowed to die. Doctors, and a judge, thought otherwise, and the baby had the surgery. It is exactly this sort of scenario that the couple in Dr Boyle's case study wanted to avoid.

In most cases, of course, doctors come to agreement on treatment - or the withdrawal of treatment - without the need to resort to the courts. But the couple's decision raises disturbing ethical questions about both the law on abortion and the rapid advances being made in treating very premature babies. What happens when a 23-week foetus is considered to be "viable" when born? Already, a handful of babies born at 22 weeks have survived, although many have been left with severe physical and mental disabilities.

Dr Boyle posits the possibility of a time when even a woman who miscarries a foetus at six weeks could see it resuscitated and given full intensive care, possibly against her wishes, because it is considered viable. Should the viability of the foetus influence the abortion limit? Recent pictures of 12-week-old foetuses moving in the womb have reignited the debate over whether the time limit for terminations should be reduced.

The law has not come before the Commons since 1990, when the time limit was reduced from 28 to 24 weeks. In the eyes of the law, "personhood" is granted at birth. But if, within the space of 24 hours, a woman can move from having the legal right to terminate her baby at 24 weeks, to having no say over its treatment if it is born at 25 weeks, have we got the balance right?