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When you’re waiting on the results of a pregnancy test, minutes can feel like hours. Test too early, however, and you risk the chance of an inaccurate result. To help ensure your test accuracy, there are some guidelines you should follow—like knowing where you are in your menstrual cycle or when you last ovulated.

Here’s everything you need to know about pregnancy tests, including how they work, when to take one for the most accurate result and symptoms that could signal it’s time to take a test.

How a Pregnancy Test Works

Pregnancy tests detect human chorionic gonadotropin (HCG), commonly known as “the pregnancy hormone.” This hormone is generated from the placenta, an organ that connects the fetus to the mother. It begins to form and protect the egg once it’s implanted along the uterine wall, signaling pregnancy has begun. At first, HCG levels are low, but they increase at the beginning of the pregnancy before leveling off.

As HCG levels increase—often doubling every three days until reaching their peak within the first 8 to 11 weeks of pregnancy—they can be detected in both blood tests (conducted at a doctor’s office) and urine tests (most often taken at home).

There are three types of at-home tests: strip, cassette and midstream. Strip tests require you to urinate into a cup and place the test strip directly into the urine. Cassette tests require you to urinate into a cup and then place drops of urine directly onto the test stick. Finally, midstream tests require you to urinate directly onto the test stick.

Your HCG levels must reach 25 milliInternational units (mIU) per millimeter (mL) to indicate a positive result—which often happens within 3 to 5 weeks after your last menstrual cycle. At-home pregnancy tests detect these HCG levels and most often reveal lines or a reading of “Pregnant” to indicate a positive result, depending on the type of test you take.

How to Check Pregnancy at Home

“Most home (urine) pregnancy tests detect HCG levels of 20 (mIU) or greater,” says Kelly Culwell, M.D.—known professionally as “Dr. Lady Doctor”—a board-certified OB-GYN and a previous medical officer for the World Health Organization in Geneva, Switzerland. “Blood tests detect HCG levels of 2 (mIU) or greater, so a blood test might be positive before a urine test is.”

While a typical urine test will show positive results at about 20 to 25 mIU, it’s dependent on your urine concentration—the less diluted the urine, the higher the concentration of HCG. Since urine is more concentrated in the morning, a test taken then is more likely to produce a positive result than one taken later in the day. Still, most urine tests will be positive by one to two days after a missed period.

Symptoms Signaling It’s Time to Take a Test

The biggest indicator it’s time to take a pregnancy test is a missed period. It isn’t the only sign, however. Symptoms that signal it may be time to take a pregnancy test, according to Dr. Merhi, include:

  • Missing a period
  • Breast fullness/soreness
  • Frequent urination
  • Nausea
  • Vomiting
  • Fatigue
  • Abdominal bloating

When Is the Best Time to Take a Pregnancy Test?

You can take a pregnancy test any time of the day. But if there’s a chance you’re early in pregnancy, take it in the morning when your urine is most concentrated so the test can more easily detect the HCG hormone, says Dr. Culwell.

“The best time in the cycle to take a test would be after you have missed a period,” says Dr. Culwell. “This will make it less likely that you might miss an early pregnancy if the HCG levels are too low to be picked up by the test.”

Nonstress Test

A nonstress test electronically monitors the fetus’s heart rate and movements. In this test, a belt is positioned around your abdomen. It is called a “nonstress” test because medications are not used to stimulate movement in your unborn baby or trigger contractions of the uterus.

Screening Tests

Your doctor may recommend other screening tests. For example:

Glucose screening can check for high blood sugar levels, which could be an indication of gestational diabetes, a form of diabetes that can develop during pregnancy. To conduct the test, which is usually performed between the twenty-fourth and twenty-eighth week of pregnancy, you’ll be asked to drink a sugar solution and then a sample of your blood will be collected. If a high level of glucose (a type of sugar used for energy) is in the blood, then additional testing should be done. This will determine if you do have gestational diabetes, which is associated with an increased likelihood of pregnancy complications.

Pregnancy Tips for Women

Group B streptococcus (GBS) screening, which will determine whether a type of bacteria is present that can cause a serious infection (such as meningitis or a blood infection) in your baby. While GBS bacteria are common and may be found in the mother’s vagina or rectum—and are harmless in healthy adults—they can cause illness if they’re passed to a newborn during childbirth. If GBS is detected in a pregnant woman, the doctor will prescribe antibiotics to be given through a vein (intravenously) during the birthing process; once the baby is born, she may be observed for a longer time in the hospital nursery. The GBS screening test is usually given between the thirty-fifth and thirty-seventh week of pregnancy.

HIV (or human immunodeficiency virus) testing is now commonly done in pregnant women, preferably early in their pregnancy. HIV is the virus that causes AIDS, and when a pregnant woman is infected with the virus, it can be passed to her baby during pregnancy, delivery of her baby, or during breastfeeding.

Other Tests

Other tests may be recommended, depending on your own physical health and personal and family history. For example, particularly for women with a family history of genetic problems or for those who are age thirty-five or older, your obstetrician may advise tests that can detect genetic disorders. The most common genetic tests are amniocentesis and chorionic villus sampling.

How soon can I take a pregnancy test?

You can take a pregnancy test anytime after your period is late — that’s when they work the best. It’s a good idea to take a pregnancy test as soon as possible if you miss your period or think you might be pregnant.

The earlier you know you’re pregnant, the sooner you can start thinking about your options and get whatever care you need to stay healthy.

Many pregnancy tests say they work a few days before a missed period, but the results are usually less accurate then. Read the label on your pregnancy test to find out when to take a pregnancy test and how accurate it’ll be.

Sometimes a pregnancy test is able to find pregnancy hormones in your urine as early as 10 days after unprotected sex. But these results aren’t super reliable, and you may get a false positive or false negative test result.

If your periods are very irregular, or you don’t get periods at all for one reason or another, your best bet for accurate results is to take a pregnancy test 3 weeks after sex.

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