How pregnancy tests work

What do pregnancy tests measure?

Pregnancy tests measure a hormone called human chorionic gonadotropin or hCG that is only produced in pregnancy. Home pregnancy tests measure this hormone in urine.

hCG is produced shortly after the embryo implants to the uterus by the cells that will form the placenta. This hormone starts being produced shortly after conception and implantation.

The main function of hCG is to maintain the progesterone production from the corpus luteum until the placenta can produce enough progesterone on its own (around 10 weeks).

hCG in urine

Regular pregnancy tests measure the presence of hCG in urine. These tests will not show specific levels of the hormone, they will only detect if it is present or not. The test strips will be exposed to the urine and react to the presence of the hCG hormone. Some tests come with 2 lines or a positive sign and some say Pregnant/Not Pregnant. In the case of the lines, there have to be 2 visible lines (control and test line) to have a positive result. The lines in the hCG pregnancy tests represent the level of hCG hormone in urine which has a 1 to 1 correlation with hCG in blood.

Home pregnancy tests are inexpensive, easy to use and give results quickly.

How to use pregnancy tests? Read here!

hCG in blood

The hCG hormone can be measured in the blood to know the exact levels. The amount of hCG can give some insights into the pregnancy. High hCG levels could indicate multiple pregnancies or some trouble with the pregnancy. Low levels could indicate that the placenta is not forming the way it should, that the egg is implanted but not in the uterus, or that the pregnancy is 11 weeks or older (hCG levels normally decline after 11-14 weeks of pregnancy).

Blood hCG tests are more sensitive than urine tests as they can detect very low levels of the hCG hormone, but they are more useful for monitoring complications or overall health of both you and the fetus.

When to take a pregnancy test?

It is important not to take the pregnancy too early. hCG levels can be very low the first days after implantation which will result in a negative pregnancy test. We know it can be hard to wait! But it is best to wait at least until your Period is late, for the most accurate results.

If the ​result is negative, pregnancy is not yet completely ruled out. You could get a negative result even though you are pregnant if you take the test too early. Maybe the hCG level in the blood was still too low, the test was applied too early, the test was not correctly done or there was too much bacteria in the urine.

Having a hard time getting through the Two Week Wait? Read this blog post!

The best time to take a pregnancy test is when the hCG hormones have reached a level above the sensitivity of your urine tests. However, to be very certain when the levels are twice the sensitivity, the pregnancy tests are definitely positive. That is 5-10 days after the missed period date or 19-24 days after ovulation. The Pearl app marks these days on your personalized ovulation chart, helping you know when to test exactly for your cycle.

Are all pregnancy tests the same?

Not all pregnancy tests are the same, some are more sensitive than others. This means that the more sensitive tests react to less amount of hCG and could detect pregnancy earlier. The most sensitive pregnancy tests react to around 20-25 mlU/mL of hCG (Pearl is in this group) and the less sensitive react to 100 mlU/mL.

The Pearl Kit comes with 2 pregnancy tests. The app will recommend you to take a pregnancy test on day 19 post-ovulation and day 24 post-ovulation after ovulation. These two days minimizes false positives. That would be roughly 3-5 days after a missing period and one week later. Doing multiple tests on the same day helps you know if the test is accurate, but doing them a few days apart helps you confirm the positive or negative result.

If you believe have pregnancy symptoms but the results are still negative, it is best you see your doctor.

Check out his blog post about the best time to take a pregnancy test.