After the troubling discovery earlier this year that Yakima County had logged an unexpectedly high number of fatal birth defects in 2012, the state Department of Health said Tuesday that its subsequent investigation yielded no common risk factors that could be the cause.

“State and local public health investigators found no significant differences between women who had healthy pregnancies and those affected by anencephaly, a rare neural tube defect,” according to a news release from the Health Department.

Anencephaly is a birth defect that is uniformly fatal. It’s caused by the failure of the protective neural tube to close completely around the spine at the base of the brain during fetal development — a process that should be complete after four to six weeks of pregnancy. The result is the absence of a large part of the brain and skull.

Based on rates determined by the Centers for Disease Control and Prevention, public health officials expected only one or two anencephaly cases among the 10,000 annual births in Yakima, Benton and Franklin counties. But in 2012, officials discovered 12 cases.

Earlier this year, health department officials said they’d found eight cases; after more rigorous investigation, where they combed through all area hospital and clinic records, they upped that total to 12.

Unfortunately, the investigation was unable to pinpoint any specific factor that led to the spike in cases.

“We looked at education; didn’t see any differences. Didn’t see any differences in BMI (body-mass index, related to obesity). We looked at insurance status, country of birth of the mother. We looked at other health factors ... We didn’t see anything” that linked the cases, said Mandy Stahre, an epidemic intelligence officer with the CDC who’s assigned to the Health Department.

Hospitals are supposed to annually report any cases of anencephaly to the state, but Stahre said that some information isn’t captured, especially if the pregnancy ends before 20 weeks, hence the department’s more intense look at hospital records.

The department also looked at previous years, finding two anencephaly cases in the tri-county area in 2011 and seven in 2010 — which is still higher than health officials expect. Data in years prior to 2010 is harder to access, Stahre said, because of outdated medical records systems.

“We’re going to continue monitoring throughout 2013. We want to see if these numbers were really just a fluke, or if we’re seeing an increase over the years,” Stahre said. “Unfortunately ... with a lot of these cluster investigations, the numbers are so small that it’s very difficult to find a difference between your cases and your healthy pregnancies.”

Health officials say the No. 1 factor for anencephaly is the folic acid intake of the mother early in the pregnancy, so they recommend all women of childbearing age take folic acid daily.

Hispanic women are at a higher risk of anencephaly, usually because of diet, Stahre said; grains found in bread are fortified with folic acid, but cornmeal is not.

Another risk factor is nitrates in the water, so the Health Department recommends pregnant women who use private well water get their well tested for nitrates and bacteria every year.

A large-scale study led by the Texas A&M Health Science Center released in June showed women with higher-than-recommended intake of nitrates in their drinking water were more likely to experience several different birth defects, including anencephaly.

• Molly Rosbach can be reached at 509-577-7728 or mrosbach@yakimaherald.com.