How Pregnancy Tests Work

The workings and limitations of home pregnancy tests

Before you decide which home pregnancy test (HPT) is best for you, it's important to understand what happens during the early stages of pregnancy and how HPTs use chemical changes in your body to detect a pregnancy.

Every month, about two weeks after a woman's menstrual period, she ovulates. Ovulation means that one of a woman's two ovaries releases a mature egg into the fallopian tube. If a sperm penetrates this egg, it becomes fertilized and begins growing and dividing rapidly from one cell to a cluster of many cells. After about six to 10 days, this cluster of cells will attach or implant to the lining of the uterus. At this point a hormone called human chorionic gonadotropin (hCG) begins to be produced by the cells that will make up the placenta in order to help maintain the pregnancy. The amount of hCG produced will increase rapidly and can be detected both in a woman's blood and in her urine. Home pregnancy tests use an antibody that attaches to hCG to detect if there is any present in her urine.

In most tests, if hCG is present, a colored line will appear on the test in addition to the control line which indicates the test is working properly. These lines appear as parallel lines on some devices, and intersect to form a plus sign on others. hCG is measured in thousandths of International Units, or mIU per milliliter (ml). For instance a test that states it can detect 25 mIU/ml means that for every milliliter of urine, it can detect 25 thousandths of International Units of hCG. The lower the number, the more sensitive the test and the earlier you can test to see if you're pregnant. The lower the amount of hCG, the more likely the test lines will appear faint and it may be difficult to decipher the results. Some tests have tried to do away with the uncertainty by producing a digital readout that says pregnant or not pregnant.

Regardless of how the results are displayed, there is a great deal of variance within a woman's menstrual cycle that can inhibit the early effectiveness of home pregnancy tests. Assuming a woman has a 28-day cycle, it is expected by the time she has missed the first day of her expected menstrual period that approximately 14 days have passed. This will have allowed enough time for the egg to be fertilized and then implant in the uterus. Though this works in theory, in the real world you can miscalculate the first day of your next period or your ovulation or implantation can be delayed.

In a 2001 study published in the Journal of the American Medical Association (JAMA), the authors found that 10 percent of the pregnant women they studied tested negative on a home pregnancy on their first day of a missed period because the fertilized egg had yet to implant. Even after a full week from their missed period, only 97 percent of women had begun producing hCG and thus 3 percent would have tested falsely negative. In a 2004 study from the American Journal of Obstetrics and Gynecology, researchers tested the urine of 25 pregnant women to see how much hCG was present at the time of their missed period. While most home pregnancy tests market a sensitivity of 25 mIU/ml, researchers discovered that to catch 95 percent of the pregnancies they studied on the first day of a woman's missed period, a test needed to have a sensitivity of at least 12.5 mIU/ml.

On the flip side, a woman can test positive only to have the test turn negative a week later. The increased sensitivity of home pregnancy tests allows women to test sooner, but by testing sooner we also detect a greater number of pregnancies that will eventually miscarry. Up to a quarter of pregnancies miscarry; the  majority of these occur within the first 12 weeks and many occur within the first few weeks. While in the past a woman may have assumed a late period to be just that, with the increased sensitivity of home pregnancy tests she may discover that she is pregnant, only to miscarry within a week's time.

Another important study evaluated what hormones are present in a woman's urine early in pregnancy. In a report published in Clinical Chemistry, scientists examined 592 urines samples and discovered that there are multiple forms of human chorionic gonadotropin (hCG) produced during pregnancy. In fact during the early weeks of pregnancy half of the total hCG is composed of a form of hCG with multiple sugars attached to it called hyperglycosylated hCG or H-hCG. While home pregnancy tests report their sensitivities to regular hCG, they do no report their sensitivities to H-hCG or the mixture of hCG and H-hCG that is commonly seen in the first weeks of pregnancy. Thankfully a 2005 study published in the Journal of the American Pharmacists Association does test numerous pregnancy tests for their sensitivities to H-hCG and the mixed hCG.