strip test tube

How Do Pregnancy Tests Work?

It is worth noting that pregnancy tests are based on the detection of the hormone hCG in the urine. As soon as your urine is brought into contact with the specially treated pregnancy test strip, you will receive a result that indicates in a very short time if the pregnancy hormone hCG has been detected or not.

How can you tell if a pregnancy test is positive?

Indeed, the majority of pregnancy tests give a positive result, as follows:

  • Two lines. For some home pregnancy tests, one line in the results window indicates that the test is negative and that you are not pregnant, while two lines mean that the test is positive and that you are pregnant. A faint positive line, on the other hand, can leave you feeling confused.
  • A plus sign. The results of some tests may be displayed either with a plus or minus symbol. A negative symbol means you are not pregnant; but if you see another line cross the negative line to form a plus sign that means you are actually pregnant. Another line will also appear in the control box indicating that the test has worked.

From the moment you are pregnant, your body starts producing hCG. This means that if hCG is detected in your urine, the test result is positive.

Some women do rush to have a home pregnancy test early in their pregnancy. They often take a test before or shortly after their first missed period. Even though hCG is present in their urine, women often produce a lower level of the hormone resulting in a positive pregnancy test with a low line. In general, these women are pregnant, but their pregnancy is not yet very advanced.

When is the best time to take a pregnancy test?

As a general rule, in a 28-day cycle, a woman usually ovulates around the 15th day. The fertilization of an egg in a normal pregnancy takes place in the fallopian tube before traveling to the uterus where it is implanted in the lining of the uterus. Soon after a fertilized egg is implanted, a woman’s body begins producing hCG from the cells of the developing placenta in the uterus. Approximately eight days after ovulation, traces of hCG can be detected early in pregnancy. By this, a woman can have positive results several days before the start of her menstruation.

The delicate moment to perform a test is that the length of the first half of a menstrual cycle is more variable than that of the second. Therefore, the duration from the first day of menstruation until ovulation can vary by several days from month to month. Sexual activity around ovulation increases the possibility of fertilization of the egg released by the sperm. But even in this case, the implantation time of a fertilized egg can vary. And hCG is only produced after implantation.

After implantation, hCG levels begin to double roughly every 48 hours. Women vary in their baseline hCG levels, hCG levels in early pregnancy, and hCG doubling time, depending on the woman and the pregnancy. In addition to the implantation date, these factors will influence how soon you can get a positive pregnancy test.

Choosing the right time for a pregnancy test is a risk-benefit calculation: a test performed earlier than 12 days after ovulation implies considering the risk of being disappointed with a possibly false result against the unlikely result of an early positive result.

False-Positive Pregnancy Test

Very rarely, you may get a false-positive result. When this happens, it means that you are not expecting a baby while the test is indicating that you are pregnant. You can get a false-positive result if your urine contains blood or protein. Also, taking certain medications, such as tranquilizers, anticonvulsants, hypnotics, and fertility drugs, can lead to false-positive results.

In such rare cases, you can get a false-positive result:

  • Some rare ovarian cysts
  • Blood or protein in your pee
  • Certain drugs, such as tranquilizers, anticonvulsants, hypnotics, and fertility drugs

However, in some cases you may get a positive result and later find out that you are no longer pregnant. This is likely to happen if you have had a chemical pregnancy or early loss, or an ectopic pregnancy. These do not represent false positives, but true positive pregnancy results, although the pregnancy does not continue.