The problem lies with variations in exactly when an early embryo may implant in a woman's body, the article explains.

Allen J. Wilcox, M.D., Ph.D., of the National Institute of Environmental Health Sciences, and colleagues studied 221 women, 21 to 42 years of age, who were planning to conceive. The women kept track of their menstrual cycles and froze urine samples for later analysis.

The authors used an extremely sensitive test to analyze the urine for human chorionic gonadotropin (hCG), a hormone that increases with embryo implantation and indicates early pregnancy.

Of 136 women who conceived, implantation was estimated to have occurred by the first day of the missed period in 90 percent.

But for 10 percent of the women who became pregnant, the hCG test obtained on the first day of the expected missed period was negative, suggesting that implantation occurred later.

"The highest possible screening sensitivity for an hCG-based pregnancy test therefore is estimated to be 90 percent on the first day of the missed period," the authors write.

"The time of implantation has its own natural variability, ranging from 6 to 12 days after ovulation," the authors state. "Thus, the exact interval from onset of previous menstrual period to ovulation and then to the detectability of a pregnancy is inherently difficult to predict."

Seven days after the first day of the missed period, test results using a very sensitive test to detect hCG showed 97 percent of the pregnancies had indeed implanted.

The authors note that home pregnancy kits, which also detect hCG, are less sensitive than the test used in their study. They cite a report indicating that the sensitivity level of most kits detects pregnancy three or four days after implantation.

The authors point out that some pregnancy kit package inserts state that if the test is negative, "you are probably not pregnant."

"This unfounded assurance could have important consequences," the authors state. "... 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."

"Better information on the limits of early testing can help balance the costs and benefits of early detection against the risks of a false-negative test result," the authors conclude.

The Early Pregnancy Study was conducted as an intramural research project of the National Institute of Environmental Health Sciences, National Institutes of Health, a research agency of the U.S. government.

(Reference: JAMA. 2001; 286:1759-1761)

10-Oct-2001