Ectopic pregnancies -- the dangerous development of fetuses outside the womb -- are increasing "at epidemic proportions," federal health experts report.

The Centers for Disease Control said the estimated 88,000 cases of ectopic pregnancies reported in 1987 -- the last 12-month period in which statistics are available -- marks a 19 percent increase in one year and more than a doubling in a decade.

The CDC report follows a study, published in the Journal of the American Medical Association in June, which said that women who have had chlamydia, a common sexually transmitted disease, are more than twice as likely as other women to have ectopic pregnancies.

Chlamydia and other sexually transmitted infections can cause pelvic inflammatory disease in women, which is believed to be the chief cause of ectopic pregnancies.

"If a woman has an infection of the fallopian tubes, they can become blocked or scarred, and the ovum does not get transferred to the uterus -- it implants outside the womb," said Hani Atrash, chief of the Pregnancy and Infant Health Branch at the CDC.

Pelvic inflammatory disease "does not explain all the increases," he said, noting other studies implicate factors such as smoking and stress.

Ectopic pregnancies have increased about fourfold since 1970, when national surveillance began, the CDC said. The 1970 rate was 4.5 per 1,000 reported pregnancies, compared with 16.8 in 1987.

"The increases are of epidemic proportions," Atrash said, adding that the steady, two-decade increase is greater than would be expected simply from improved reporting.

Previous studies have reported that the rate of ectopic pregnancies is highest for women over 30 and for minorities.

Thirty American women died as a result of ectopic pregnancies in 1987. While this is a relatively small number, it accounts for 10 percent of all maternal deaths in the U.S., Atrash said. If detected early, ectopic pregnancies can be ended surgically, eliminating the danger to the mother.