How Does a Pregnancy Test Work?

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 a 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.

How Soon After Conception Is the Test Effective?

Many tests can start to detect pregnancies as early as 10-15 days after conception or one day after a missed menstrual period. But since the level of hCG continues to rise, the test becomes more sensitive over time. So on the first day after a missed period, the urine pregnancy test may only detect some pregnancies, but one week later it would likely detect most pregnancies. If you tested and the result was negative, and you still have not started your period, wait a few days and test again.

How Do I Use an In-Home Pregnancy Test?

It is important that you 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 usually take about five minutes to display.

How Accurate Are They?

In-home pregnancy tests are very accurate if used appropriately 1 week after your missed period. However, if used incorrectly, if the instructions are not followed, or if you use the test too early, then the results are much less accurate.

It is important that you follow the directions and understand how to interpret the results. Manufacturers state that in-home pregnancy tests can be used as soon as one day after a missed period, but understand that the tests are much more accurate if you wait a week after a missed period.

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. 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.

If your home pregnancy test is positive for a pregnancy, it is very likely to be correct. It is extremely rare that a test would give you a positive result if you were not really pregnant.

Other Possible Causes of a Missed Period

The most common cause of a missed period is pregnancy. But what if you have missed a period and are not pregnant? Some other causes of missed periods include but are not limited to:

• 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.
• Medicines—Certain drugs can cause changes to your monthly cycle. When you are prescribed a new medicine, 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 your menstrual cycle stopping.
• 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.