Home pregnancy tests are pretty reliable — in fact, most of them can detect a pregnancy with 97 percent accuracy as early as the day after you miss your menstrual period. The tests work by indicating whether or not there's a pregnancy hormone called human chorionic gonadotropin (hCG) in your urine. If you've got a measurable level of hCG, congratulations! You're pregnant — with these few, rare exceptions.
Chemical pregnancy
The most common explanation for a "false positive" is that you really were pregnant when you took the test. "Some women can get a positive pregnancy test and then three days after their period is due, they have a really heavy period," says Lanalee Araba Sam, M.D., an ob-gyn in Fort Lauderdale. "Really what they've had is an early miscarriage called a chemical pregnancy, and a lot of people call that a false positive. They don't recognize it as a positive pregnancy test that ultimately results in miscarriage." Dr. Sam explains that as many as 25 to 40 percent of pregnancies may end as an early often-undetected miscarriage. Some women don't even know they're pregnant in the first place.
Once you get your positive pregnancy test result, call your doctor and make an appointment. It may be many weeks away, and it can be stressful to wait for acknowledgment that everything's okay with your pregnancy. But, Dr. Sam explains, an ultrasound can't help your doctor see the gestational sac until about 6 weeks, and can't detect a fetus with a heartbeat until around 6 or 7 weeks. Once you see that heartbeat, your chance of miscarriage plummets to less than 5 percent.
In the meantime, look out for heavy bleeding or intense cramping, and call your doctor right away if you experience either of them.
Recent miscarriage
If you've had a miscarriage, hCG can stay in your bloodstream for several weeks afterward, or until your period returns, says Dr. Sam.
Pregnancy test errors
Check your pregnancy test kit to make sure it hasn't expired, and follow the instructions carefully. Making a mistake can affect the reliability of the test results.
Medications
As part of infertility treatments, some women receive hCG injections to cause them to ovulate. If you're one of them, your doctor will advise you on how to accurately test for pregnancy.
Other medications, such as methadone, could also interfere with pregnancy test accuracy, says Sean Daneshmand, M.D., an ob-gyn based in San Diego and founder of the nonprofit organization Miracle Babies.
Rare tumors
In some instances, tumors in the body can produce hCG. This is the case for certain ovarian germ cell tumors and gestational trophoblastic disease, a rare condition in which the cells that normally form the placenta develop tumors instead, says Dr. Daneshmand.
The bottom line is this: If you had a positive pregnancy test, it's 97 percent certain that you're pregnant.
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