A study into several cases of a rare birth defect in three Eastern Washington counties has found no common cause.

The Washington state Department of Health says state and local public health investigators found no significant differences between women who had healthy pregnancies and those affected by anencephaly, a fatal birth defect.

Anencephaly results from incomplete formation of the brain during the first month of pregnancy. Typically, one or two cases would be expected in about 10,000 annual births, but an investigation found about 8 cases per 10,000 births in Yakima, Benton and Franklin counties.

The Health Department said Tuesday that the higher than expected number could be coincidental, but that health officials will continue to monitor births in the region through 2013 to see if more can be learned.