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A POPULATION‐BASED CASE‐CONTROL STUDY OF ANENCEPHALUS AND SPINA BIFIDA IN A LOW‐RISK AREA

For the period 1973 to 1977, a total of 536 cases of anencephalus and spina bifida were ascertained in Los Angeles County, California, a low‐risk area, and compared with a 2 per cent random sample of all live births in the county. Women with Spanish surnames had an elevated risk for anencephalus and to a lesser extent for spina bifida; Blacks were at lowest risk, especially for spina bifida. The occurrence of a previous fetal death was a strong risk factor for anencephalus but there was no association between socio‐economic status and either of the defects. Advanced maternal age was a stronger risk factor for spina bifida than for anencephalus but, as in other studies in low‐risk areas, no increased risk was observed among teenage mothers. Paternal age did not show any independent association after controlling for mother's age. Finally, the association of high birth‐order with the birth defects was weakened greatly after simultaneous adjustment for age and ethnicity.