1. They may not provide a reliable reading as soon as you think.

While many home pregnancy tests claim to detect pregnancy even before you miss a period, that’s not always the case. The tests work by detecting the level of human chorionic gonadotropin (hCG) in your urine, which your body produces once the fertilized egg implants in your uterus. “Only a small amount of hCG is released at first, so it may take a few days for it to reach a level that a test can measure,” Dr. Scott says. “The result is more likely to be accurate if you wait to test until after you’ve missed a period.” If you do decide to test early and get a negative result, it’s a good idea to take another a few days later just to be safe.

It’s also important to know that while many home pregnancy tests claim to be 99 percent accurate, that’s only true if you follow the instructions to the letter. “In the hands of a healthcare provider, urine tests are between 97 and 99 percent accurate, but for the everyday person it’s probably closer to 75 percent because it’s easier for her to make mistakes,” Dr. Scott explains.

2. The time of day you test really does matter.

Manufacturers recommend women test their urine first thing in the morning for good reason. “Your urine is more concentrated then because you haven’t had anything to drink for several hours, so hCG is much easier to detect,” Dr. Scott says. Testing any later in the day could lead to a false negative if you’ve had a lot to drink because your urine will be more diluted.

Other common errors to be aware of include using an expired test or one that was stored in a hot, damp location (like your bathroom), checking the results too soon (most tests require you to wait five minutes), and not using enough urine.

3. False negatives are more common than false positives.

There are very few scenarios that can lead to a false positive. “Anyone who’s using injectable fertility medication containing hCG is better off going to their doctor to get a blood test to see if they’re pregnant because it’s more accurate than a urine test,” Dr. Scott says. In very rare cases, cancers or other types of tumors could trigger a false positive. “Waiting too long to check the results of your test could also result in a false positive, depending on the quality of the test,” Dr. Scott adds.

What if you get a positive reading but the lines or symbols are very faint? Chances are you’re pregnant. “The darkness of the reading is related to how much hCG is in your urine, so it may appear lighter the earlier you are in your pregnancy,” Dr. Scott explains. When in doubt, perform another test a few days later to confirm the results, or opt for digital pregnancy tests if you’re worried about your ability to interpret ones that use lines or symbols.

4. Pricier tests aren’t better.

Cost typically varies based on how you test your urine, not accuracy. Those that have you collect your urine in a cup (and then dip the stick in it or use a dropper to put it in a different container) are usually more expensive than those that require you to hold the stick in your urine stream. “It really comes down to personal preference,” Dr. Scott says. “All pregnancy tests are pretty good.” The only types she recommends avoiding are those that claim to estimate how far along you are in your pregnancy: “They simply aren’t that accurate.”