Home pregnancy tests miss early detection

The majority of home pregnancy tests do not detect most pregnancies if used within the first day or two of a missed period. Home pregnancy tests measure levels of human chorionic gonadotropin (hCG) in the urine. Despite the claim by many tests that they can be used on the first day of a woman's missed period, researchers at the University of New Mexico at Albuquerque found that, among 18 tests, only one - First Response, Early Result - appeared to be sensitive enough to detect pregnancy in at least 95 percent of women on the first day of their missed periods. However, all tests became more accurate over time, suggesting that home pregnancy test users should wait until at least one week after their missed period to get a more reliable result.

During the study, researchers measured the amount of hCG in the urine of 25 pregnant women on the first days of their first missed periods, and tested 18 brands of home pregnancy tests to determine how well they detected those levels of hCG. They found that on the first day of a missed period, the amount of hCG present in most women's urine was so low, only the First Response test appeared capable of detecting pregnancy in more than 95 percent of women. Only 3 of the 18 tests produced a positive result when testing urine that contained the amount of hCG typically present during the second day after a woman's first missed period. By the third day, 8 tests appeared effective.

Researchers explained that many pregnancy tests fail to detect pregnancy because most of the hCG present in the body during the initial days of pregnancy is in a modified form, for which the tests have only a weak affinity. The proportion of total hCG made up of the modified form decreases slowly over the course of pregnancy, however, and the tests therefore become more effective over time.

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