When a pregnancy test is done, usually between 15 and 20 weeks, it is important to know the gestational age of the unborn baby, the age of the mother, her weight, her ethnicity, whether she is diabetic, and whether it is a twin pregnancy. If any one of these factors is reported inaccurately, a false positive or false negative result may occur.
After conception, the hormone hCG (human chorionic gonadotropin) is produced by the placenta and may be found in the blood and urine. Home pregnancy tests are used on urine samples.
For best results, carefully read the directions on each individual test, as instructions may vary — and accuracy depends on you following the directions precisely. If there are terms you don’t understand, look them up or contact the manufacturer — most provide toll-free numbers. In general, be sure to store the test at room temperature, make sure the test has not expired, and do not open the inner package (containing the test device) until you are ready to test.

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.