Pregnancy tests often claim to be accurate as early as 1 day after a missed period. However, studies show that at least 10% of the time this is not true.

Pregnancy tests work by detecting human chorionic gonadotropin (HCG), a hormone normally produced only in pregnancy. HCG can be measured in blood or urine. Blood tests are more sensitive, but the accuracy of a pregnancy test also depends on other factors.

A woman's body begins to produce HCG when the fertilized egg implants into the wall of the uterus. The timing of the events that lead up to implantation vary greatly. Ovulation usually occurs 14 days before the menstrual period, but even in women with very regular cycles, the timing may vary by a couple of days. The next event, fertilization, can also vary, depending on the timing of intercourse. Finally, the time between fertilization and implantation can be between 6 and 12 days.

Other factors that determine the accuracy of a pregnancy test include:

• the brand (some are more sensitive than others)
• the time of day you test your urine (first thing in the morning is best)
• the care with which the test is performed (instructions must be followed precisely)

Given all these factors, a urine test for pregnancy will almost always be accurate a week after your expected period. If you need to know sooner than that or are still uncertain, your doctor can send you for a blood test.