Home pregnancy tests are wrong a lot more often than their manufacturers admit, a medical technology expert reports.

"Manufacturers should be encouraged to reevaluate their claims," Mary L. Doshi writes in the American Journal of Public Health after evaluating three of the do-it-yourself urine tests.

Doshi asked 109 women to take one of three tests -- Answer, Daisy 2 and e.p.t. -- between six and 20 days after missing a period. Accuracy ranged from 46 percent to 89 percent, with accuracy defined as "agreement with the actual outcome: pregnant, not pregnant."

Manufacturers claim accuracy averaging 97.4 percent.

In general, the more time that has since a period was missed, the more accurate the test. The tests look for human chorionic gonadotropin (HCG), a hormone present in pregnant women. "It is crucial to point out," Doshi writes, "that the concentration of HCG in urine at nine days after the missed menses is far less than the hormone concentration at 15 days.

Only Daisy 2 claims to be accurate less than nine days after the missed period.

False negative results -- failure to detect a pregnancy -- are the most troublesome, Toshi writes, because women might delay getting prenatal care. There is also the risk that a pregnant woman, thinking she is not pregnant, might get an X-ray, which could be harmful to a fetus.

False positives -- detection of pregnancy when the women is not really pregnant -- can be caused by vibration of the test tube, allowing too much time for the test, or a foreign substance in the test tube. While professional laboratories can compensate for such errors, she writes, "the consumer does not have this safeguard."