These 4 factors could all cause a pregnancy test to be wrong

At the core, home tests are designed to pick up traces of human chorionic gonadotropin, or HCG a hormone that is produced during pregnancy. HCG is made by cells formed in the placenta, in order to help grow the fertilized egg after it has attached to the uterine wall, according to . "Like all hormones, HCG has several functions in the body," says Heather Bartos, M.D., ob-gyn. "The hormone is produced by placental cells and promotes the corpus luteum, a normal ovarian cyst in pregnancy, which secrets progesterone. Progesterone is necessary for a healthy pregnancy. HCG may also have an effect of helping prevent immune reactions toward a developing fetus."

Pregnancy tests are advertised as 99 percent accurate, if done correctly. In short: a woman pees on the end of a dipstick and her urine comes into contact with a specially treated strip made to detect if HCG is present. In minutes, results are available via positive/negative symbols or pregnant/not pregnant text. But, sometimes other factors affect the results of even the most reliable kits.

During pregnancy, HCG levels increase daily. If a woman thinks she's getting a false positive result, an immediate visit to the doctor is imperative. Blood tests should be taken, and administered again 48 hours later. Ultimately, the origin of elevated HCG in the blood needs to be determined. If it's due to pregnancy, these specific hormone levels will double within two days time, says Bartos. Blood tests can detect pregnancy sooner, and more accurately, than at-home tests.

Here are four reasons why the stick you’ve peed on may be misleading, and therefore getting a second opinion might be best.

All cancers produce some kind of hormone, and certain cancer cells produce beta HCG, which might sway pregnancy test results. “Certain medical conditions, such as ovarian tumors, can lead to elevations in the HCG hormone," says Bartos. "For instance, choriocarcinoma, a malignant cancer involving retained placental cells, produce [HCG] in high amounts, just as if it were in a pregnancy."

This is more likely to cause a false negative than a false positive, but it's still worth nothing. “Urine pregnancy tests are good, but even these can fail," says Bartos, specifying that HCG is at its highest level when urine is fresh a.k.a. in the morning. "If the first-morning urine isn’t used, the pregnancy hormone level may not be high enough to discover it on the test."

Should a woman become pregnant but suffer a miscarriage, there are still hormones in the body that will allude to carrying a child. “There’s something called a chemical pregnancy—think false start—that leads to a positive test, but nothing ever happens afterwards,” says Bartos. An egg implants in the uterus, and HCG is produced by the cells that would have developed into the placenta. "The most common explanation for a false positive is that you really were pregnant when you took the test, but it wasn’t viable. This is caused by a chemical pregnancy, which occurs if a fertilized egg, known as the embryo, stops growing very soon after conception,” says Scott Capobianco, M.D., ob-gyn at Mission Hospital in Mission Viejo, California.

Over-the-counter pills like Advil won’t affect pregnancy test results, but some prescription drugs might have a strong impact. “False positives can also occur if you take a pregnancy test too soon after taking a fertility drug, or other medications (like HCG shots) that contain the hormone human chorionic gonadotropin," says Capobianco. "Although it’s not medically indicated, some patients take HCG for weight loss in conjunction with a weight-loss management program. That involves injections of the hormone."