For women waiting to find out if they are pregnant, a few days can feel like an eternity. The desire to know for sure often leads many women to take a home pregnancy test. What's more, some home pregnancy tests claim to give accurate results up to three or four days before a woman expects her period. But just because a woman can test earlier doesn't necessarily mean that she should.

Most know how a pregnancy test works – urine is placed on the absorbent tip and after a few minutes, a positive or negative reading appears. But why does this happen? What is it that the test measures to determine whether or not a woman is pregnant?

"Home pregnancy tests work by measuring the amount of human chorionic gonadotropin (hCG) in a woman's urine, a hormone that is produced by pregnant women," says Dr. Shari Brasner, a clinical instructor at Mount Sinai School of Medicine. "hCG levels begin to appear and increase daily once a fertilized egg implants in a woman's uterus, which on average can range from six to 12 days after ovulation. Because of this natural variability, the day a woman's hCG levels are detectable by home pregnancy tests varies as well."

If hCG levels begin to increase at conception, testing for pregnancy early simply lets you know earlier, right? According to Dr. Brasner, the exact opposite is true. "Testing too early can often be worse than not knowing at all," she says. "A 'false negative' test result typically occurs when a pregnant woman tests too soon and does not yet have enough hCG in her system. So while the test reads negative, she is in fact pregnant."

"I actually took a pregnancy test before I missed a period – about 25 days into my cycle," says Darla Meriss of Chicago, Ill. "I was indeed pregnant, but the line was very, very faint – barely visible. So to be sure, I took another one a few days later. I was pregnant."

According to Dr. Brasner, an overwhelming number of doctors think early pregnancy detection is not a good idea. "Many doctors don't like the idea of a woman taking a pregnancy test at all before she actually misses a period," says Dr. Brasner. "Nearly four out of five doctors – 79 percent – polled in a survey commissioned by Pfizer Consumer Healthcare at the American College of Obstetricians and Gynecologists (ACOG) meeting prefer that women not use a home pregnancy test up to four days before their expected menstrual period. Clinical testing shows almost half of pregnant women – 48 percent – will get a false negative response four days before their period is due."

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