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Early Home Testing Fails to Detect Some Pregnancies

WebMD Health News

Oct. 9, 2001 -- Your first home pregnancy test says it ain't so. You may be glad; you may be sad -- but don't pour yourself a glass of wine just yet. Even though the label says "99% accurate," there may still be a baby on board.

It's not that the tests aren't reliable. They are. But most of them claim to be able to detect a pregnancy as soon as the first day of a missed period. That's too soon to be sure, according to a report in the current issue of TheJournal of the American Medical Association.

The problem is that many of the tests say they can be used as early as the first day of a missed period. Even when used this soon, the instructions usually tell a woman she is not pregnant if the test is negative. This may lead some women to resume use of alcohol, medicines, or other substances that can harm a developing baby. Some women may return to workplaces where they are exposed to toxic substances.

The earlier you test, the more likely it is you will get a falsely negative test result, study leader Allen J. Wilcox, MD, PhD, tells WebMD. "That is the trade-off," he says.

"The advantage to testing early is if a woman is pregnant -- and the pregnancy has progressed to the point where the test can detect it -- she will have useful information. The disadvantage of early testing is that you might not be able to believe a negative result."

Wilcox is a senior investigator at the National Institute of Environmental Health Sciences in Durham, N.C. His research team set out to learn exactly how well pregnancy tests work at the earliest time a woman is likely to use them: on the first day of a missed period.

The researchers didn't test home pregnancy kits. Instead, they used a laboratory test that is 100 times more sensitive than the home tests. They repeatedly tested 136 women who planned to have children. When they tested a woman on the day she expected her period, one out of every 10 negative tests was wrong.

It wasn't that the test didn't work -- it was just too early to detect every pregnancy.

Many of the women tested positive on the pregnancy test a week or more before their expected period. But a few women didn't test positive until a week or more later.

"There is no clear right or wrong answer when to test," Wilcox says. "If a woman starts testing on the first day of her expected period, she has at least a 10% chance of missing a true pregnancy."

A woman should keep on testing every couple of days after her expected period until she either gets a positive result or starts to menstruate. It's important not to take a negative test too seriously. The only sure sign that a woman is not pregnant is her period.

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