Research shows earliest testing does not reveal all pregnancies

October 09, 2001

CHAPEL HILL -- Commercially available pregnancy tests kits may not be accurate on the first day or so after a missed period because of natural variability in ovulation and when developing embryos attach themselves to the lining of women's wombs, a new study concludes.

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." Attachment occurs roughly nine days after fertilization, but ranges from six to 12 days, Wilcox said. Pregnancy cannot be detected before implantation.

"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."

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. A report on the findings appears in the Oct. 10 issue of the Journal of the American Medical Association.

Besides Wilcox, adjunct professor of epidemiology at the UNC School of Public Health, authors are Drs. Donna Day Baird, David Dunson, Ruth McChesney and Clarice R. Weinberg. McChesney is a Columbia University faculty member, while Baird, Dunson and Weinberg all are with NIEHS and UNC as adjunct faculty.

"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.

"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. U.S. 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.
UNC News Services

Note: Wilcox can be reached at (919) 541-4660 or 667-1688 or via e-mail at
News Services Contact: David Williamson, (919) 962-8596
School of Public Health Contact: Lisa Katz, (919) 966-7467

University of North Carolina at Chapel Hill

Related Public Health Articles from Brightsurf:

COVID-19 and the decolonization of Indigenous public health
Indigenous self-determination, leadership and knowledge have helped protect Indigenous communities in Canada during the coronavirus disease 2019 (COVID-19) pandemic, and these principles should be incorporated into public health in future, argue the authors of a commentary in CMAJ (Canadian Medical Association Journal)

Public health consequences of policing homelessness
In a new study examining homelessness, researchers find that policy such a lifestyle has massive public health implications, making sleeping on the street even MORE unhealthy.

Electronic health information exchange improves public health disease reporting
Disease tracking is an important area of focus for health departments in the midst of the COVID-19 pandemic.

Pandemic likely to cause long-term health problems, Yale School of Public Health finds
The coronavirus pandemic's life-altering effects are likely to result in lasting physical and mental health consequences for many people--particularly those from vulnerable populations--a new study led by the Yale School of Public Health finds.

The Lancet Public Health: US modelling study estimates impact of school closures for COVID-19 on US health-care workforce and associated mortality
US policymakers considering physical distancing measures to slow the spread of COVID-19 face a difficult trade-off between closing schools to reduce transmission and new cases, and potential health-care worker absenteeism due to additional childcare needs that could ultimately increase mortality from COVID-19, according to new modelling research published in The Lancet Public Health journal.

The Lancet Public Health: Access to identification documents reflecting gender identity may improve trans mental health
Results from a survey of over 20,000 American trans adults suggest that having access to identification documents which reflect their identified gender helps to improve their mental health and may reduce suicidal thoughts, according to a study published in The Lancet Public Health journal.

The Lancet Public Health: Study estimates mental health impact of welfare reform, Universal Credit, in Great Britain
The 2013 Universal Credit welfare reform appears to have led to an increase in the prevalence of psychological distress among unemployed recipients, according to a nationally representative study following more than 52,000 working-age individuals from England, Wales, and Scotland over nine years between 2009-2018, published as part of an issue of The Lancet Public Health journal on income and health.

BU researchers: Pornography is not a 'public health crisis'
Researchers from the Boston University School of Public Health (BUSPH) have written an editorial in the American Journal of Public Health special February issue arguing against the claim that pornography is a public health crisis, and explaining why such a claim actually endangers the health of the public.

The Lancet Public Health: Ageism linked to poorer health in older people in England
Ageism may be linked with poorer health in older people in England, according to an observational study of over 7,500 people aged over 50 published in The Lancet Public Health journal.

Study: Public transportation use linked to better public health
Promoting robust public transportation systems may come with a bonus for public health -- lower obesity rates.

Read More: Public Health News and Public Health Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to