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).

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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.

“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.

You should wait to take a pregnancy test until the first day of your missed period. Since HCG is only present once implantation of the egg occurs, there often isn’t enough of the hormone to be detected until you miss your menstrual cycle.

“Pregnancy tests pick up the hormone secreted after implantation, which usually occurs about two weeks after the sperm meets the egg,” says Dr. Culwell. “For women who have regular periods, this usually means that HCG can be detected as soon as you miss a period.”

This, of course, assumes you have a regular menstrual cycle that can be easily tracked and predicted. If this isn’t the case, you may choose to instead track ovulation, or when an egg is released and makes its way to the uterus where it can potentially be fertilized. An at-home ovulation predictor kit can help you track fertilization.

If this method is used, it’s best to take a pregnancy test “approximately 14 days after a documented ovulation,” says Zaher Merhi, M.D., an OB-GYN, reproductive endocrinologist and founder of Rejuvenating Fertility Center with locations in New York and Connecticut.

Signs You Should Take A Pregnancy 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.”

Where To Buy A Pregnancy Test

You can buy a pregnancy test at most drug stores and grocery stores, or even online.