Women with a family history of diabetes who are free from the disease themselves are more likely to develop pregnancy-related diabetes, a new study confirms.

And the risks associated with having a brother or sister who is diabetic are much higher than having one or even two parents with the disease, Dr. Catherine Kim of the University of Michigan Medical School in Ann Arbor and her colleagues found.

The increased demands placed on the body during pregnancy can cause some women to develop abnormally high blood sugar. The condition, known medically as gestational diabetes, typically gets better after a woman delivers her baby, but it increases her risk of developing type 2 diabetes later on.

The more relatives a person has with type 2 diabetes, the greater their risk of developing the condition themselves. But little is known about how a woman’s family history of the condition affects her risk of developing gestational diabetes.

To investigate, Kim and her team looked at 4,566 women participating in the National Health and Nutrition Examination Survey, all of whom had at least one child. Ninety-seven percent had never been diagnosed with diabetes, about 1 percent had gestational diabetes only, and 2 percent had type 2 diabetes.

Having a mother or father with diabetes increased the likelihood of having diabetes or gestational diabetes to a similar degree, the researchers report in the American Journal of Obstetrics and Gynecology. But while having two parents with diabetes boosted the likelihood of having diabetes eight-fold, this only doubled the likelihood of gestational diabetes.

On the other hand, having a diabetic brother or sister increased gestational diabetes risk more than seven-fold, but only slightly upped type 2 diabetes risk.

“The odds of [gestational diabetes] increased most markedly when a sibling was affected,” Kim and her team write. And when the researchers accounted for early-life factors such as education and poverty, the risk associated with having a diabetic sibling actually increased. “Sibling-only history may be a greater risk factor than previously documented,” they say.

The findings suggest, the researchers say, that gestational diabetes may follow a different pattern of inheritance than type 2 diabetes, which is closely associated with being overweight or obese.

Further investigation of these patterns could help identify women who are at particularly high risk of developing type 2 diabetes after having the gestational form of the condition, they add, “and thus target them for future prevention interventions.”

SOURCE: American Journal of Obstetrics and Gynecology, December 2009.