An epidemic of dangerous pregnancies in which the fetus develops outside the womb is afflicting expectant mothers, many of whom have had a common and often symptom-free sexually transmitted infection or who douched, researchers say.

The number of such pregnancies, called ectopic pregnancies, increased fourfold from 1970 to 1985, affecting 78,400 women in '85, researchers reported in Wednesday's Journal of the American Medical Association.

A woman's risk of developing an ectopic pregnancy, which causes 11 percent of maternal deaths, more than doubles if she has had the bacterial infection chlamydia trachomatis, the researchers said.