Research Shows Earliest Testing Does Not Reveal All Pregnancies
"The interesting thing we were able to look at was how the time of implantation varies among women in relation to their expected menstrual period," he said. "While on average, most people will implant before their expected menstrual cycle, the natural variability is something that doctors and pregnancy kit manufacturers haven't taken into account."
Ten percent of pregnancies in an original group of 221 women were undetectable that day even when an especially sensitive method was used to detect the telltale human chorionic gonadotropin hormone, researchers say. Pregnancy test kits from drugstores are reasonably reliable, but they are not as precise as the test employed by the North Carolina scientists.
"Our subjects were healthy women who were planning to get pregnant," said Dr Allen J Wilcox, senior investigator at the National Institute of Environmental Health Sciences and a University of North Carolina at Chapel Hill faculty member. "By analyzing daily urine specimens while they were trying to get pregnant and throughout the earliest part of pregnancy, we were able to pinpoint the day of implantation, which is the day the fertilized egg attaches itself to the lining of the mother's uterus."
The new study shows clearly that 10 percent of the embryo attachments -- and hence new pregnancies -- had not occurred by the day women expected their periods to begin, the physician said.
"These findings mean is that if women were to buy a test kit, follow the directions and test themselves on the first day of their expected periods, the test would be negative for many women, and they would not know that they'd become pregnant," Wilcox said. "This is true even if their doctors had given them a blood test for pregnancy that day."
If women waited for another week, the 10 percent of false negatives would decline to three percent or less, he said. A report on the findings appeared in the October 10, 2001 issue of the Journal of the American Medical Association.
"The message is that women who test themselves could be misled into thinking they're not pregnant because the package inserts imply pretty strongly that if the tests are negative, then pregnancy has not occurred," Wilcox said. "In fact, there is at least a small chance that they are pregnant, but the pregnancy hasn't got to the point yet where it is detectable."
The new findings are helpful, he said, because so many women buy the pregnancy test kits. In 1999, they bought about 19 million over-the-counter kits in the United States alone. US sales totaled about $230 million, and inserts typically instruct women to test as early as the first day of a missed period.
"Unfounded assurance that a woman is not pregnant could have important consequences," Wilcox said. "For example, women with a negative test result may fail to protect themselves from exposures to toxicants in the workplace or to medications that could damage a developing embryo. Many women will test positive a week or more before their period is expected, while a few women will test positive only a week or more afterward."
Young women and teens are frequent users of test kits but might be especially prone to false-negative test results because they are more likely to ovulate late.
Subjects, who ranged in age from 21 to 42, were recruited in central North Carolina between 1982 and 1986. Of the original group, 151 were pregnant for at least six weeks, and all had kept menstrual diaries and froze urine samples daily. Of those, 136 provided information about their usual cycle length and formed the final study group.
Wilcox and colleagues analyzed the samples for human chorionic gonadotropin, a hormone that increases rapidly following implantation, with an extremely sensitive immunoradiometric assay. That assay is more than 100 times more sensitive than most commercial pregnancy test products.