A home pregnancy test is a quick and easy way for a woman to find out if she might be expecting. The do-it-yourself kits sold at drug stores and supermarkets check a woman's urine and can detect a pregnancy in its early weeks not long after she misses her period.
Home pregnancy tests are marketed to women as a tool to use to see if they are pregnant, said Michele Helgeson, a certified nurse midwife and director of midwifery at Brigham and Women's Hospital in Boston. If positive results are obtained, it can be a springboard to seek out medical attention from their health provider for early prenatal care, she said. [Related: Early Signs of Pregnancy]
An earlier diagnosis can also provide health professionals with the opportunity to advise women about their pregnancy options and to discuss avoiding potentially harmful habits that can affect the developing fetus, such as smoking, drinking alcohol and using illegal drugs.
Previously, pregnancy tests were done by doctors and nurses, and they were more complicated. They involved getting a urine sample from the woman, injecting it into a rabbit, frog or female mouse, and then examining the animal's reproductive system to measure human chorionic gonadotropin (hCG) levels, a hormone found only in pregnant women.
The first home test kits were approved by the FDA and widely marketed in the United States in the late 1970s. The test consisted of purified water, a test tube containing sheep red blood cells and a medicine dropper. It also relied on getting a urine sample to test for hCG, and it took two hours to obtain the results.
A home test protected a woman's privacy about her sexual activity, and it also made it quicker and more convenient for her to figure out whether she might be pregnant early on in the process.
These days, there are dozens of brands of home pregnancy tests, and they typically sell for $15 to $20 for two tests. Pregnancy test kits are among the top-selling medical products for home diagnostic testing with sales in the United States reaching $228 million in 2012.
What the test measures
Home pregnancy tests measure hCG levels in the urine. This hormone, found only in women once a pregnancy is achieved, is produced by the placenta shortly after a fertilized egg implants in the uterus and it enters the bloodstream and urine.
In early pregnancy, the amount of hCG doubles in the blood every 48 to 72 hours, and then its levels plateau during the second trimester, Helgeson told Live Science.
When to test
"To have the best results, it's probably better for women to wait about one week after a missed period to do the test," Helgeson said. This leaves a broader window, especially for women who have irregular menstrual cycles that are typically shorter or longer than a 28-day cycle.
How they work
The test is done by collecting a urine sample and dipping a test strip into the cup or placing some drops of it onto a test strip. It can also be done by holding a test strip in the urine stream as a woman goes to the bathroom.
Tests recommend using midstream urine, or peeing first into the toilet and then into the cup to collect the sample. Results are fast and often obtained in a few minutes.
Some manufacturers tout claims of 99 percent accuracy if tests are used on the first day of a missed period but some research disputes these claims.
Depending on the test, results may be given as a plus or minus sign, a single or double line, a color change, or a digital readout saying "pregnant" or "not pregnant." Some tests contain a control window that can be used as a comparison with the results window.
Medications containing the pregnancy hormone hCG, such as some fertility treatments, may affect pregnancy test results and produce a false-positive reading. This is a test says that a woman is pregnant when she is not.
But most medications, such as birth control pills, pain relievers or antibiotics, will not interfere with results nor will drinking alcohol or using recreational drugs but these behaviors are not advisable if trying to become pregnant.
Depending upon when a woman has her first prenatal visit, it's customary for some medical practices to repeat a urine pregnancy test at their offices or do a blood test, Helgeson said. She said that if a woman is further along in her pregnancy, such as in her eighth week or beyond, her health care provider may do an ultrasound instead to determine the viability of the pregnancy and her due date. "This is determined on an individual basis," Helgeson explained.
Home tests vs. tests at a doctor's office
The urine-based pregnancy tests done in a provider's office is similar to the one that's done at home and is based on the same principle, hCG levels, Helgeson said. She explained that a blood test for pregnancy, known as the beta hCG test, trumps a urine test because it is a more sensitive indicator of hCG.
A blood test can detect a pregnancy earlier than a urine test, by detecting low levels of hCG in blood as early as six to eight days after ovulation. But a blood test can only be performed in a doctor's office or clinic, and it might take longer to get results, according to womenshealth.gov.
Tips for using home pregnancy tests
- For best results, read package directions and follow them, especially instructions about how and when to do the test, and waiting enough time to read the results. Be sure to use the test before its expiration date posted on the package, and store the kit in a dry place at room temperature.
- A woman should use her first morning urine to do the test because it has more hCG in it. Urine becomes more dilute as a person drinks more liquids during the day, Helgeson said. Although not all tests recommend using the first morning urine, it will contain the highest levels of pregnancy hormone in the sample.
- If a woman suspects she is pregnant and gets a negative test result, repeat the test several days to a week later because hCG levels rise rapidly during early pregnancy. Testing too close to conception or too early after a missed period may produce incorrect results because the placenta has not yet had enough time to produce enough hCG to be detected by the test.
- If a woman has questions about the product, its correct use or whether studies have been done on the test's accuracy, phone or online contact information for the manufacturer is often listed on the box.