Pregnant or not pregnant? It’s the eternal question that leads many women to turn to home pregnancy testing. It helps to know in advance how these pregnancy tests work so you can use them effectively.
Home pregnancy tests measure for a hormone called human chorionic gonadotropin, or HCG, in your urine. This hormone is made by the tissue that becomes the placenta, which nourishes a growing fetus, says Dr. Mary Jane Minkin, a clinical professor of obstetrics, gynecology and reproductive services at Yale University in New Haven, Connecticut.
The hormone is produced early on in pregnancy, after a fertilized egg attaches to the wall of the uterus, says Dr. Lauren Demosthenes, of Greenville, South Carolina, who is an OB/GYN and senior medical director for Babyscripts, a company focused on improving health during pregnancy.
Home pregnancy tests work by placing a urine test strip under your stream of urine or by collecting urine in a cup and placing the test strip in that cup. The tests have different ways to show you’re pregnant. Depending on the brand, the test will display a colored line, a symbol or the words “pregnant” or “not pregnant.”
While home pregnancy tests simply measure the presence of HCG in urine, blood tests at your health care provider can measure the actual level of HCG. Blood tests are more sensitive and can detect HCG about 10 days after conception. Because they can measure the amount of HCG present, the results help determine if a pregnancy is proceeding normally.
Most home pregnancy tests claim that they’re 99% accurate — blood tests also are 99% accurate — but there are some ways you can misread the results of a home pregnancy test or get a false result, depending on when you take the test and how you perform the pregnancy test.
Here are seven things you should know about home pregnancy testing to ensure you get the most accurate results.
7 Tips for Pregnancy Testing
1. See a doctor before you get pregnant.
If you know you want to have a baby, the best time to see a doctor isn’t after you have a positive test — although you should see a doctor then too. See a doctor before pregnancy to review your health and learn what changes you should make to ensure a healthy pregnancy, Demosthenes advises.
Some changes you’ll likely discuss include:
— Changing any medications to avoid ones that aren’t safe during pregnancy.
— Getting diabetes or high blood pressure under better control if needed.
— Avoiding drinking and smoking.
— Maintaining a healthy weight.
— Adding extra folic acid to lower the risk of birth defects like spina bifida.
2. Know how soon you can use the test.
If you take a test too soon, you may still have a low level of HCG and you’ll get a negative result, even if you’re pregnant. “Some women are upset that they can’t tell they are pregnant the exact moment they conceive,” Minkin says.
It takes time for the sperm to reach the fallopian tube and then meet up with an egg that’s ovulated. Ovulation occurs when a fertile egg is released from one of the ovaries.
Generally, ovulation occurs 14 days before your period. If an egg is fertilized by sperm somewhere in the fallopian tube, that egg then travels to the uterus and implants in the lining. The placenta develops to feed the growing embryo, secreting HCG. There won’t be enough HCG to detect by a home pregnancy test until a week or so after fertilization, Minkin explains.
Many tests are designed to detect pregnancy after a missed period, but some can show if you’re pregnant six to eight days after ovulation, which is even before a missed period, Demosthenes says.
However, because you may not know when you have ovulated, you could end up taking the test too soon and getting a negative result. That’s why, for accurate results, the ideal time to take a pregnancy test is a week after the first day of your missed period, to ensure you have enough HCG for the test to detect. If you have irregular periods, then you should wait until 14 days after you’ve had sex.
[READ: The Fertility Diet.]
3. Take the test first thing in the morning, if possible.
That’s when your urine is most concentrated, so it will contain more HCG. Later in the day, when you’re hydrated and your urine is more diluted, the levels of HCG will be lower. Of course, buying a pregnancy test in the afternoon and then forcing yourself to wait and take the test the next morning is tough. If you can’t wait until your first morning bathroom visit, it’s fine to take it another time of day. The only way the results would be affected is if you’re overly hydrated and your urine is particularly diluted.
4. Know when to read the results.
Read the packaging to find out when the test will display results. This generally means three to five minutes — which will feel like hours. There’s also a Goldilocks effect, though. If you wait too long, you can get an inaccurate reading. The test may look positive because of an evaporation line left by the urine, Demosthenes explains.
Some pregnancy tests now allow you to scan your test result with a smartphone app to help you read the test more accurately, Minkin says.
5. It’s fine to buy a less expensive test, but always read the packaging.
Some of the pricier home pregnancy tests may have a slight advantage because they can sync with an app to help you interpret the results or they’re more sensitive, allowing you to use them earlier. No matter which test you buy, what’s more important is to read the box and any test instructions, says Dr. Kecia Gaither, director of perinatal services at NYC Health + Hospitals/Lincoln in New York City.
By reading the packaging, you can find out:
— The exact purpose of the test. There are ovulation tests that look similar to pregnancy tests, so you’ll want to make sure you get the correct item.
— How accurate the test is. Most pregnancy tests are 99% accurate.
— How to use the test.
— How soon you can expect results.
— Whether or not the test is expired. Don’t use an expired test as you may not get accurate results.
6. There are times when the results may be false.
For instance, if you’re taking HCG as a fertility treatment, you could end up with a false positive result. Some pituitary and ovarian tumors can also lead to a positive HCG test because they secrete HCG.
Also, there are times when you can have a positive test and be pregnant, but this doesn’t mean the pregnancy will be normal. You could have a miscarriage or an ectopic pregnancy, which is a pregnancy that isn’t implanted in the uterus and could cause harm if it’s not treated early with medication or surgery, Demosthenes says.
It’s also possible to take the test too soon, before your body produces enough HCG. That would cause a negative result even if you eventually become pregnant. If your urine is too diluted, this could also lead to a negative result. If you think you’re pregnant but get a negative result, try taking the test a couple days later.
If you have irregular periods, you may have trouble detecting if you’re pregnant. Out of 100 women, 10 to 20 won’t detect a pregnancy on the first day of a missed period, according to the FDA.
7. See your primary health provider or an OB/GYN once you have a positive test.
During your visit, the doctor can confirm your pregnancy with a blood test. Blood tests can be used earlier on during pregnancy and can measure how much HCG is present, which helps monitor whether the pregnancy is progressing normally.
HCG levels double in your body every two to three days, so finding out your exact HCG level can ensure it’s increasing appropriately, especially if you have spotting or another concern, Minkin says.
Blood test results can take a couple of hours or up to a day, Gaither says. During the visit, the provider also will discuss any vitamins you should take, timing for follow-up appointments and genetic testing.
Nowadays, telehealth and app-based education can help a lot with early pregnancy care, Demosthenes says. Apps such as Babyscripts can help you track your pregnancy and provide information on prenatal and postpartum care.
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Update 05/07/21: This article was previously published and has been updated with new information.