The darkness of the result line on pregnancy tests varies according to hCG level. Both the left and middle First Response tests are positive; the one at the left resulted from urine with a higher hCG concentration. The test on the right is negative. In all cases, the upper line indicates that the test has worked.
The main reason is that in 10 percent of pregnant women, the embryo does not implant until after the first day of the missed period, according to Dr. Wilcox's findings. Using urine tests to measure hCG concentrations in 136 normal early pregnancies during a study of possible environmental causes of early pregnancy loss, Wilcox found that even as late as a week after the missed period, 3 percent of pregnancies still hadn't implanted.
"Until implantation, it doesn't matter how sensitive the test is," Wilcox says. "You can't detect the pregnancy before it's producing the stuff that you're measuring, which is hCG."
Even pregnancies that have implanted may produce too little hCG for many at-home test kits to detect the day after the missed period when read after the waiting period specified in test package instructions. Some kits improve in detection when read after a wait of 10 minutes, but waiting longer than that may produce a negative result that looks faintly, misleadingly positive.
