Testing for a protein in the urine of pregnant women accurately predicts whether they will develop preeclampsia, a potentially deadly condition that affects one in 20 U.S. pregnancies, a study said on Tuesday.

The finding may pave the way for a simple screening test that could allow doctors to treat the condition before it develops into eclampsia, an uncontrollable rise in blood pressure that threatens the life of the mother and her fetus, researchers said.

Preeclampsia is the rise in blood pressure that precedes eclampsia.

Eclampsia, in which blood vessels feeding the placenta constrict instead of expanding as they should, is one of the world’s leading causes of maternal and infant mortality.

The condition usually occurs after the 20th week of gestation and can cause seizures, kidney failure or stroke in the mother and kill or cause premature birth of the fetus.

Doctors try to detect preeclampsia by monitoring the blood pressure of expectant mothers, but by the time pressure becomes elevated, it may be too late.

Researchers have previously proposed other tests to predict preeclampsia, but these have ultimately turned out to be flawed.

Five years ago, Harvard Medical School researcher Ananth Karumanchi discovered that depressed levels of the protein placental growth factor and elevated levels of another protein heralded the onset of preeclampsia.

In his study, screening on stored urine specimens from 120 women who developed preeclampsia and 118 women who had normal pregnancies showed the former had a sharp drop in levels of the placental growth factor protein six to eight weeks before the onset of symptoms.

To gauge its effectiveness, screening for the protein was also performed on urine from pregnant women who developed two other conditions - gestational hypertension and delivery of a small-for-gestational-age infant. There was no similar drop in levels of the protein.

“A simple urine test could help predict the onset of this disease (preeclampsia) one to two months before the onset of clinical symptoms and that could make a tremendous difference in outcomes for patients, in particular those women who have limited access to specialized medical care,” Karumanchi said.

The study appeared in the Journal of the American Medical Association.