Women who suffer from high blood pressure during their pregnancies have higher levels of a hormone that increases blood pressure in the tissue that connects mother and fetus, researchers say.

Their study was published online Friday in the journal Hypertension.

Pregnancy-induced high blood pressure is called pre-eclampsia. It affects 2.6 per cent of pregnancies in Canada, according to Health Canada, and leads to high levels of protein in the urine and swelling of the face and hands. It can also cause a pre-term delivery of the baby and can be fatal for both the infant and mother.

When they tested 21 pregnant women with pre-eclampsia, researchers at Wake Forest University Baptist Medical Center in Winston-Salem, N.C., found high levels of the hormone angiotensin II in part of the placenta that supplies food and oxygen from the mother to the fetus. Angiotensin II constricts blood vessels, causing blood pressure to increase.

The researchers believe that this constriction is what causes less oxygen and nutrients to be delivered to the fetus, leading to a lower birth weight and other complications.

In the 25 women studied without pre-eclampsia, tissue samples revealed that this hormone was present in much lower levels.

"This finding may be part of the pre-eclampsia puzzle," said Lauren Anton, a graduate student and lead author of the study, in a release. "Anything that gets us closer to understanding this disease is important because there is no treatment and no cure and women are still delivering babies too early."

The researchers hope that their discovery will lead to further research and perhaps a treatment for pre-eclampsia.