A pregnancy test reacts to human chorionic gonadotropin (hCG), the hormone that your body starts producing when the embryo attaches to the uterine wall. This happens between days six and 10 after fertilization.
Sensitive tests detecting hCG in urine may indicate pregnancy on the first day of a missed period, while the majority of home tests detect it on days five to 10 from your expected period.
A blood test for hCG can show a positive result earlier than the most sensitive urine test. Home pregnancy tests that detect hCG in urine also have different sensitivities.
On the test package, it will say 10, 20, or 25 mIU/mL. The smaller the number, the higher the sensitivity of the test. You can use our hCG calculator to work out your hCG levels at home.
The level of hCG is indicated by blood test results. To interpret your results, doctors refer to medical tables showing the correspondence between the hCG value and a certain time in pregnancy.
Initially, hCG doubles every two to three days and reaches its maximum by weeks nine to 10 of pregnancy. At-home urine tests for hCG only detect its presence, not the amount. Thus, they cannot indicate the baby’s gestational age.
The surest way to get an accurate pregnancy test is to follow the manufacturer's recommendations. Midstream tests do not require collecting urine. Instead, put the appropriate end of the test under your urine stream for a few seconds and wait until the result appears.
Alternatively, the test strips are dipped into a container of collected urine. Cassette tests involve collecting urine in a cup and then pipetting a few drops of it onto a test strip.
Cassette and digital home pregnancy tests can exclude the errors of analyzing and interpreting the results.
There is no need to guess what a faint line means because either a plus sign (+) / minus sign (–) or "pregnant" / "not pregnant" will appear on the screen.
Your first-morning urine, which accumulates in the bladder overnight (after more than four hours of continuous sleep), contains the maximum amount of hCG.
Therefore, most health care providers recommend taking a test in the morning. Extremely diluted urine due to excessive drinking can make the line too faint or give a false-negative result.
A faint line may appear due to an insufficient urine level of hCG. It can also occur if you did not follow the instructions.
In most cases, even a faint second line on a home test indicates pregnancy, if it has been carried out correctly. To confirm your results, you can:
To avoid false-negative and false-positive results from a home pregnancy test, make sure to follow the instructions, store the package at the right temperature, and interpret your results after the correct amount of time.
Also, make sure not to use expired test kits.
A home pregnancy test can yield a false-positive result for several reasons:
If you aren’t sure about the results of the test, take another one in a few days.
Several factors can lead to a false-negative home pregnancy test:
Try retesting in a few days.
If you are taking fertility medications containing hCG, the results of a home pregnancy test may be affected.
Alcohol consumption, lactation, menopause, and the intake of most medicines usually will not affect the test results.
A pregnancy test reacts to the hCG that is produced by the embryo and released into the pregnant person’s bloodstream.
For an ectopic pregnancy, the implantation site (e.g., the uterine wall or another organ) does not matter. Ectopic pregnancy can be signaled by blood spotting and/or pain in the lower abdomen, as well as by the slow increase of hCG in the blood.
If you suspect an ectopic pregnancy, be sure to consult a health care provider.
After a positive test result, your health care provider will complete several blood tests to monitor the changes in your hCG level. A slow increase doesn't necessarily indicate a problem, but it requires additional clarification.
Normally, the amount of hCG should double every 48–72 hours or increase by 60 percent or more every two days.
If your hCG levels don’t double, it may indicate a nonviable pregnancy, whether intrauterine or extrauterine.
A rapid increase of hCG in the blood can indicate, among other things, twins or triplets.
“Doing a Pregnancy Test.” NHS Choices, NHS, www.nhs.uk/pregnancy/trying-for-a-baby/doing-a-pregnancy-test/.
Betz, Danielle. “Human Chorionic Gonadotropin.” StatPearls [Internet]., U.S. National Library of Medicine, 30 Aug. 2020, www.ncbi.nlm.nih.gov/books/NBK532950/.
“Pregnancy Test: MedlinePlus Medical Test.” MedlinePlus, U.S. National Library of Medicine, 3 Dec. 2020, medlineplus.gov/lab-tests/pregnancy-test/.
“Home Pregnancy Tests: Can You Trust the Results?” Mayo Clinic, Mayo Foundation for Medical Education and Research, 12 Jan. 2019, www.mayoclinic.org/healthy-lifestyle/getting-pregnant/in-depth/home-pregnancy-tests/art-20047940.
