Date of conception and prevention of neural tube defects.

The authors studied the time of conception of 280 fetuses and liveborns with anencephaly or spina bifida seen in a French Canadian population during a period of six years. There is an evident seasonal variation in the conception of NTD fetuses. Using a wave statistical model applied to data recorded over a six year period the authors have rejected the "no variation between the six two-month intervals" hypothesis at a P less than or equal to 0.005 level. It is postulated that, in any vitamin intake or environmental program to prevent NTD in families at risk, one must take into account the annual and seasonal variations, ethnic groups identity and the date of conception in the choice of control mothers.