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6 Things Every Woman Should Know About Pregnancy Tests

Having tons of choices for pregnancy tests is great, but the sheer number of brands to choose from is dizzying—not to mention trying to figure out how they work and which one works best. What’s the difference? How should I test for best results? And how do these things work, anyhow? Read on to find out.

First things first: There may be dozens of different home pregnancy tests on the market, but whether you’re in the doctor’s office or buying whatever’s on sale at the grocery store, all urine pregnancy tests measure a hormone called human chorionic gonadotropin, or HCG. HCG is in your system even without pregnancy, but once a fetus is implanted and starts to grow, the level of hormone—measured in what’s called International Units, which just means the volume of hormones—in your blood and urine takes off. “Once the level of hormones gets to 25 International Units [IUs], the test will appear positive,” says G. Thomas Ruiz, M.D., ob-gyn at the Orange Coast Memorial Medical Center in Fountain Valley, CA.

Ever wonder why pregnancy test instructions advise you to test first thing in the morning? That’s because after several hours of sleep, HCG has a chance to build up in your urine as it’s stored in your bladder. The urine is more concentrated with HCG and therefore more likely to result in a positive test if you’re pregnant, since at least 25 IUs of HCG are needed for that test to turn positive. But the tests available on the market today are much more sensitive than they were even a decade ago, says Ruiz, so testing in the morning is “not as necessary” as it might have been with older tests. Basically: For best results, test in the morning—but don’t stress if you can’t.

If you’re agonizing over whether you should grab a test from the Dollar Store or splurge on a digital test at Target, Ruiz says there’s virtually no difference between the two. “You don’t have to buy a super expensive test for it to be accurate,” he says. “Get whatever is on the shelf at Costco.” The reason for this is because all home pregnancy tests measure the same thing at basically the same threshold of 25 IUs of HCG. Although some tests may have a lower threshold, Ruiz claims that testing below 25 IUs increases your chances for a false positive, since a small amount of HCG (typically less than 10 IUs) is in your system normally. Bottom line is that any test will do.

All urine tests are qualitative, says Ruiz, meaning they only indicate the presence of HCG in urine after it hits 25 IUs. But there are also quantitative tests, which are performed by measuring the amount of HCG in a person’s bloodstream. In a woman who is at risk of miscarriage or other pregnancy complications, doctors may want a qualitative test to see whether the HCG is increasing as the pregnancy matures. “We expect a doubling of HCG every 48 hours,” says Ruiz. “Once it gets to 1800, we should be able to see a pregnancy in her uterus. Once it gets to 2500, we should be able to see a fetal pole and possibly a heartbeat.”

Certain medications, usually those used for fertility, have HCG and can potentially raise the level of hormone in your urine enough to give you a positive pee result without a pregnancy. Another rare way to get a false positive? Pituitary disorders or germ cell cancers—tumors in the testes or ovaries—can cause elevated HCG as well.

One thing Ruiz tells all his patients? Trust the test. “Everyone repeats the test, even if they get a positive, because they don’t believe it the first time. Some people will test three times and then come to the doctor and do another test—but we use the same tests at the doctor’s office!” He says. Moral of the story? “If it’s positive twice, you’re pregnant!”

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