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Here's a key lesson that your sex ed course might’ve skipped: the 101 on pregnancy tests. For instance: What's the best time of day to test? What if there's only a faint line? And do pregnancy tests expire?

We asked two experts for answers to these questions: Dr. Julia Simon, an OBGYN and assistant professor of Obstetrics and Gynecology at the University of Chicago and Dr. Pamela Berens, professor of Obstetrics and Gynecology at McGovern Medical School at UTHealth Houston. Here’s what they had to say.

There are two main types of pregnancy tests: urine and blood tests. Sex ed recap: Your body produces a hormone called human chorionic gonadotropin (HCG) after a fertilized egg is implanted in the uterus. The tests analyze your blood or urine for the presence (and level) of that hormone. 

Your doctor will order a blood/serum test and this will be performed at a lab. These tests are around 99% accurate and can detect even small amounts of HCG. They’re sometimes used to confirm the results of an at-home test. But according to Dr. Berens, an at-home test can usually suffice. A doctor might order a blood test if you’re facing symptoms such as bleeding or abdominal pain, which are also symptoms of a miscarriage. 

There are three types of at-home urine pregnancy tests to choose from:

​​Midstream: Urinating directly on the test stick (like First Response or Clear Blue )

Cassette: Urinating into a cup, then applying drops of urine to the test stick (like Accu-Test or Equate First Signal One-Step )

Strip: Urinating into a cup, then putting the test strip directly into the urine (like First Response Pregnancy Comfort Check or Easy @ Home )

According to Dr. Berens, most of these tests are similar and can usually detect a pregnancy a week after a missed period with 97-99% accuracy . It comes down to what type of test you would prefer to take and which method you’re most comfortable with. 

Yes. Tests usually last one to three years after manufacturing. Over time, the antibodies that coat at-home pregnancy tests begin to lose effectiveness and evaporate. Be sure to check the expiration date on your pregnancy test before you take it. Because you’re more likely to get an inaccurate result (usually a false negative) if you use an expired test. 

Tip: Storing your test at temps between 36 and 86 degrees Fahrenheit can help slow down the expiration process.

You can take a pregnancy test any time of day . But the test may be more accurate first thing in the morning, according to Dr. Berens. “There’s typically more HCG present at that time,” she said. Thing to know: “If your urine is very diluted, the test may not detect lower HCG levels.” (Your urine can get diluted if you drink a lot of water or other liquid before you take the test).

Note: It’s also important to read the test within the recommended time period after taking it, said Dr. Simon. If you leave it out too long, the result can be inaccurate. 

No matter how faint the line is, it means you’re pregnant. But according to Dr. Berens, a faint line can mean different things for your pregnancy. It could just mean you're early in your pregnancy (think: if you test around the time your period should start). If you take the test again in a few days, you should see a stronger line. Because your body has had more time to build up more of the HCG hormone the test is trying to detect. 

But be on the lookout if you continue to see a faint line on additional tests. But if the faint test is persistent when repeated, if you’re having bleeding or lower abdominal pain, or your period is late by a few weeks, this may also be a sign of an abnormal pregnancy. And you should contact your health care provider right away, said Dr. Berens. 

A faint line can also be a sign of a false positive. But that’s unlikely. This only happens less than 1% of the time. But if you aren't sure about your results, follow up with your doctor to confirm. 

A pregnancy test can determine a major life moment. Be sure to pick the best test for you and follow the instructions carefully. And talk to your doctor if you’re unsure about your results. 

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