WOMEN planning to get pregnant can now ask their GPs to prescribe them folic acid supplements in order to reduce their chances of having a baby with spina bifida. A government-appointed committee reported as long ago as December 1992 that such supplements would be a useful preventive measure. But only recently has the Department of Health made them available on prescription. About 800 babies a year are born with spina bifida, many of whom end up in institutional care. Yet little publicity has surrounded the production of the new supplement and details do not appear in the doctors' drugs manual MIMS or the British National Formulary, making it hard to prescribe.