BOSTON, JULY 27 -- A new test revealed that most miscarriages happen during the first month of pregnancy, long before most women realize conception has occurred.

"We can say that the majority of early pregnancy loss appears to be unrecognized," said Dr. Allen J. Wilcox, who directed the study.

His work suggests that 22 percent of all pregnancies end spontaneously in the first month. Another 9 percent result in miscarriages after the pregnancy becomes apparent.

The study confirmed what many scientists have long suspected: The female body discards a large percentage of its fetuses in the first month or so after conception.

Wilcox said the body probably rejects fetuses because they are defective. When researchers study later miscarriages in which fetal tissue is recovered, they find that half show gross genetic abnormalities.

"It's reasonable to speculate that the same would be true in these very early losses," Wilcox said.

The study found that 31 percent of all pregnancies ended before birth, but the true rate of miscarriage is probably even higher, since the test missed miscarriages during the first week after conception.

An editorial published with the study in the Thursday issue of the New England Journal of Medicine speculated that half of all pregnancies may end in miscarriage.

Wilcox, a researcher at the National Institute of Environmental Health Sciences in Research Triangle Park, N.C., based his findings on a newly available test that spots a pregnancy hormone.

The study was conducted on 221 healthy young women who had decided to stop using birth control and get pregnant. The researchers collected urine and checked it for traces of chorionic gonadotropin, a hormone produced by the fetus.

This substance shows up in a woman's urine when the fetus attaches itself to the wall of her uterus, usually about a week after conception.

Wilcox noted that many physicians tell women to resume birth control for a few months after a miscarriage. But he said this may not be necessary, at least after very early pregnancy losses.

Wilcox said the miscarriages did not mean that a woman could not have children, and almost all of those who had unrecognized miscarriages were able to get pregnant again within two years.

Dr. A. Brian Little of Royal Victoria Hospital in Montreal said the test, which is not yet commercially available, may allow doctors to pinpoint more accurately the causes of infertility.