YAKIMA, Wash. -

The state Health Department is trying to find the cause of an unusually high number of rare birth defects in south-central Washington.

There were 7 cases of the defect called anencephaly in 2013 in Yakima, Benton and Franklin counties for a rate of more than eight cases per 10,000 births. That's four times the national rate.

The Yakima Herald-Republic reports health investigators are checking records and interviewing families and doctors. They're asking what the mothers ate, talking about pesticides, and looking at family histories; but so far there's no common factor.

Anencephaly is a fatal defect that occurs when the spinal cord fails to develop at the base of the brain.