Neural-tube defects and mid-cycle abstinence: a test of the 'over-ripeness' hypothesis in man.

The prevalence of mid-cycle abstinence at the time of conception of infants born with neural-tube defects was found to be only 6-7 per cent (8/119) compared with that of 9-8 per cent (32/325) at the time of conception of non-affected siblings. This is suggestive evidence against mid-cycle abstinence--and, by inference, over-ripeness--as an important possible cause of neural-tube defects.