Should a new-born baby's life be preserved at all costs-even if the quality of that life is going to be painful, with severe retardation and with enormous strain on the family that supports it? Spina bifida is one of the most common birth defects. Sometimes it has only a small effect on the child; it can be disastrous. The decision whether or not to preserve the lives of some of the worst cases has caused deep controversy in medicine. What causes it? Two years ago the potato was a prime suspect. and might still hold the clue as to the solution. Can it be prevented? In the last few months a triumphant experiment in an Edinburgh laboratory has shown that thousands of women need not fear that they might give birth to a crippled spina bifida child. Robert Reid looks at the work which is trying to unravel one of medical science's most intriguing but deadly riddles. Research FISHER DILKE Producer ROBERT REID