Facts you should know about pregnancy tests
Home pregnancy tests are a cheap and effective way of learning when you should see the doctor about your pregnancy.
*Facts you should know about pregnancy tests medically edited by: Charles Patrick Davis, MD, PhD
- Pregnancy tests work by detecting a hormone in a woman's blood or urine: hCG (human chorionic gonadotropin) is produced when an egg implanted in the uterus.
- There are two types of pregnancy tests, one uses blood and the other uses urine (home tests); a quantitative blood test detects hCG earlier than a qualitative blood or urine test.
- Most home pregnancy tests detect hCG in a urine stream when, after a few minutes, a line or symbol appears if the test is positive (indicating pregnancy); most urine pregnancy test instructions urge a second test in a few days no matter what were the first results; follow package instructions – if a control indicator does not appear, the test result should not be trusted.
- Home tests are accurate, some better (more sensitive in showing early pregnancy) than others.
- Home pregnancy tests may test positive on the first day of a missed period, but almost all detect pregnancy 1 week after a missed period.
- Call your doctor for an appointment if you test positive for pregnancy with a home test.
- It is possible to be pregnant and have a negative home pregnancy test; that is why you should wait about a week to repeat the test. Most medicines and other compounds (alcohol, drugs) do not interfere with pregnancy tests; however, any drug containing hCG (some fertility drugs) may give false test results.
- Any questions or problems with pregnancy tests should be discussed with your doctor.
How soon can I use a home pregnancy test?
Some home pregnancy tests are more sensitive than others and can be taken before your missed period. But you may get more accurate results if you wait until after the first day of your missed period.
This is because the amount of the pregnancy hormone, called human chorionic gonadotropin, or hCG, in your urine increases with time. The earlier you take the test, the harder it is for the test to detect the hCG.
hCG is made when a fertilized egg implants in the uterus. This usually happens about 10 days after conception (when the man's sperm fertilizes the woman's egg).
My pregnancy test says I am pregnant. What should I do next?
If a home pregnancy test shows that you are pregnant, you should call your doctor to schedule an appointment.
Your doctor can use a blood test to tell for sure whether you are pregnant. Seeing your doctor early in your pregnancy also means you can begin prenatal care to help you and your baby stay healthy.
My pregnancy test says I am not pregnant. Could I still be pregnant?
Yes, it is possible you could still be pregnant. It's possible to be pregnant and to have a pregnancy test show that you are not pregnant.
The accuracy of home pregnancy test results varies from woman to woman because:
- Each woman ovulates at a different time in her menstrual cycle.
- The fertilized egg can implant in a woman's uterus at different times.
- Sometimes women get false-negative results when they test too early in the pregnancy. False negative means the test says you are not pregnant when you are.
- Problems with the pregnancy can affect the amount of hCG in the urine.
If a test says you are not pregnant, take another pregnancy test in a few days. If you are pregnant, your hCG levels should double every 48 hours. If you think you are pregnant but more tests say you are not, call your doctor.
The first sign of pregnancy is most often:
What if I can't tell whether my pregnancy test is positive or negative?
Sometimes it can be hard to tell whether the test is positive or negative. The line may be faint, or you may worry whether you peed too much or too little on the stick.
No matter how faint the line or plus sign, if you see it, you are most likely pregnant. The faintness of the line can mean you are early in your pregnancy and your hCG levels are still low.
Also, the pregnancy test should have a control line that tells you whether the test was done correctly. If the control line is blank, then the test did not work and you should take another test.
What should I consider when buying a pregnancy test?
- Cost. Home pregnancy tests come in many different types. Most stores sell them over the counter (without a doctor's prescription). The cost varies depending on the brand and how many tests come in the box.
- Accuracy. Most tests can be taken as soon as you miss your period. Some newer, more expensive tests say they can be used 4 or 5 days before your period. Even so, they claim the best accuracy only after the date of your expected period.
What are the different types of pregnancy tests?
Pregnancy tests check for the hCG hormone in two ways:
- Urine test. This type of pregnancy test can be done at home or at a doctor's office.
- Blood test. This type of pregnancy test can only be done at a doctor's office. It takes longer than a urine test to get results, but it can detect a pregnancy earlier than a urine test (about 10 days after conception, compared to typically 2 weeks or more for a urine test). Your doctor may use one or both types of blood tests:
- A quantitative blood test (also called a beta hCG test) that measures the exact amount of hCG in your blood. It can find even tiny amounts of hCG. It can also tell you and your doctor how many weeks you are pregnant.
- A qualitative hCG blood test checks to see whether the pregnancy hormone is present or not. The qualitative hCG blood test is about as accurate as a urine test.
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How do I use a home pregnancy test?
All home pregnancy tests come with written instructions. Depending on the brand you buy, the instructions may vary:
- You hold a stick in your urine stream.
- You pee into a cup and dip the stick into it.
- You pee into a cup and then use a dropper to put a few drops of the urine into a special container.
Different brands tell you to wait different amounts of time, although most are around 2 minutes. Depending on the brand of the test, you may see a line or a plus symbol, or the words “pregnant” or “not pregnant.” A line or plus symbol, no matter how faint, means the result is positive.
Most tests also have a “control indicator” in the result window. This control line or symbol shows whether the test is working properly. If the control line or symbol does not appear, the test is not working properly.
Look for the toll-free phone number on the package to call in case of questions about use or results.
How accurate are home pregnancy tests?
Most home pregnancy tests claim to be up to 99% accurate. But the accuracy depends on:
- How you use them. Be sure to check the expiration date and follow the instructions. Wait up to 10 minutes after taking the test to check the results window. Research suggests that waiting 10 minutes will give the most accurate result.
- When you use them. The amount of hCG or pregnancy hormone in your urine increases with time. The earlier you take the test, the harder it is for the test to detect the hCG. Most home pregnancy tests can accurately detect pregnancy after a missed period. Also, testing your urine first thing in the morning can boost the accuracy.
- Who uses them. Each woman ovulates at a different time in her menstrual cycle. Plus, the fertilized egg can implant in a woman's uterus at different times. Your body makes hCG after implantation occurs. In up to 10% of women, implantation does not occur until after the first day of a missed period. This means home pregnancy tests can be accurate as soon as 1 day after a missed period for some women but not for others.
- The brand of test. Some home pregnancy tests are more sensitive than others. For that reason, some tests are better than others at detecting hCG early on. Talk to your pharmacist about which brand may be best for you.
I have irregular periods and don't know when my next period will start. When should I take a pregnancy test?
Most pregnancy tests claim to be the most accurate after a missed period. But irregular periods can make it hard to predict when to take the test.
Periods are considered irregular if:
- The number of days between periods is either shorter than 21 days or longer than 35 days
- The number of days in the menstrual cycle varies from month to month. For example, your cycle may be 22 days one month and 33 days the next month.
If you have irregular periods, try counting 36 days from the start of your last menstrual cycle or four weeks from the time you had sex. At this point, if you are pregnant, your levels of hCG should be high enough to detect the pregnancy.
If your test says you are not pregnant, but you still think you may be pregnant, wait a few more days and take another pregnancy test. Or, call your doctor for a blood test.
Can anything affect home pregnancy test results?
Yes. If you take medicine with the pregnancy hormone hCG as an active ingredient, you may get a false-positive test result. A false positive is when a test says you are pregnant when you are not.
Some examples of medicines with hCG include certain medicines for infertility. If you are taking medicine to help you get pregnant, you may want to see your doctor for a pregnancy test.
Most medicines should not affect the results of a home pregnancy test. This includes over-the-counter and prescription medicines such as birth control and antibiotics. Also, alcohol and illegal drugs do not affect pregnancy test results.
How do pregnancy tests work?
All pregnancy tests work by detecting the pregnancy hormone, hCG, in the urine or blood. This hormone is present only when a woman is pregnant. If the pregnancy test detects hCG, it will say you are pregnant.
hCG is made when a fertilized egg implants in the uterus. This usually happens about 10 days after conception (when the man's sperm fertilizes the woman's egg). The amount of hCG builds up quickly in your body with each passing day you are pregnant.
So if you take a home pregnancy test too soon after implantation, your hCG level may not be high enough to detect the pregnancy. If the test says you are not pregnant, take another pregnancy test in a few days.