Whether it is a blood or urine test, all pregnancy tests work the same way—by testing for the presence of human chorionic gonadotropin (HCG). HCG is the hormone produced in the placenta after egg fertilization. Its concentration doubles every two or three days, peaking around the eighth week of pregnancy. In-home pregnancy kits test urine for HCG.

Many tests advertise that they can detect pregnancies as early as 10-15 days after conception or one day after a missed menstrual period. To ensure the highest level of accuracy, experts recommend waiting 5 days after your first missed period before using the test.

Testing too early may lead to an inaccurate result; you may not have enough HCG in your body to get a reading. If you tested and the result was negative, and you still have not started your period, wait a few days and test again.

Most importantly, read the directions. Not all in-home pregnancy kits are used in the same way. With some kits, you will urinate in a cup and then use a dropper to place a small sample of your urine on the test area. Other kits require you to urinate directly on the test stick. The results, indicated by a color change, usually take about five minutes to display.

According to their manufacturers, in-home pregnancy tests are accurate 97% to 99% of the time. But researchers have determined that this accuracy rate does not hold up in "real world" situations. A study published in the Archives of Internal Medicine reported an accuracy rate of 75% when women performed and evaluated the tests on their own. The main problem, the researchers concluded, was that many women had trouble understanding the instructions.

When an error does occur, more often than not, it is a false-negative—meaning the test says you are not pregnant when you are. False-positives are rare. If the test result is negative but you are experiencing early signs of pregnancy, it is best to see your doctor. The earlier that you begin prenatal care, the better it is for both you and your baby.

The most common cause of a missed period is pregnancy. But, what if you have missed a period and are not pregnant? On October 10, 2001, the Journal of the American Medical Association listed other possible causes of a missed period:

• Birth control pills—some women who use this form of contraception may not have periods. In addition, stopping your birth control pills can lead to 3–6 months of missed or abnormal periods.
• Breastfeeding—although this can cause missed periods, you can still get pregnant without periods when you are breastfeeding.
• Medications—certain drugs can cause changes to your monthly cycle. When you are prescribed a new medication, ask your doctor about possible effects on your menstrual cycle.
• Illness—some illnesses can disrupt your period. Once you are healthy, you should return to your normal cycle.
• Eating Disorders, malnutrition, and low body weight—these may cause a decrease in the production of estrogen. Inadequate estrogen production can result in the cessation of the menstrual cycle.
• Excessive exercise—this can disrupt the production of hormones and stop menstruation.
• Menopause—as you near menopause, your period will become irregular and eventually stop.