Home pregnancy tests measure the small amount of hormones that the body produces when a fertilized egg is implanting and beginning to grow. Specifically, they contain antibodies that can pick up the presence of human chorionic gonadotropin (hCG) in pee.

Two types of hCG are important in early pregnancy. The first, hCG, is a hormone responsible for forming the mechanism by which the placenta gives nutrients to the fetus. The second, hyperglycosylated hCG (or hCG-h), is a whole different, unrelated molecule.

The two hCGs differ in when they’re present in pee during early pregnancy. In very early pregnancy (weeks three and four) only hCG-h is present, Cole said.

Regular hCG is not produced until you’re about four to five weeks pregnant. Amounts of this hormone double roughly every 72 hours in early pregnancy, reaching their peak around eight to 11 weeks, then level off for the remainder of the pregnancy. Levels of hCG in pee can vary a lot as well. If you’re pregnant and it’s been three weeks since the start of your last period, you can have anywhere from 5 to 50 mIU/mL in your pee. (That’s in milli-International Units per milliliter, a standardized unit that doesn’t really tell you much. Regardless, that’s how levels of this stuff are measured.) At six weeks past your last menstrual period, this spread is from 1,080 to 56,500 mIU/mL. According to studies by Cole and other researchers, hCG is present at low levels in the body even when a person is not pregnant, so the tests can’t be too sensitive or they might erroneously register a false positive.

False negatives, where you are pregnant but get a negative reading on a pregnancy test, are much more common than false positives.

Another way you might get a false positive is if you have what’s called a chemical pregnancy. This is a pregnancy that fails to develop beyond the earliest stages, often resulting in a miscarriage around the time menstrual bleeding would be expected.

False negatives, where you are pregnant but get a negative reading on a pregnancy test, are much more common. According to Dr. Serna at Cedars-Sinai, this can happen when you take a test too early and it can’t pick up on the hCG levels in your pee yet. If you think you may be pregnant and take the test prior to a missed period and get a negative result, wait a week and take the test again. To obtain the most accurate results, test after missing an expected period. The longer you wait after a missed period, the more accurate the test becomes. (To be sure a home pregnancy test is functioning properly, look for what’s called a control line: This should appear whether or not a second line—indicating hCG detection—appears.)

For the most accurate pregnancy testing, visit your doctor.