The aptly named Clearblue Advanced Pregnancy Test with Weeks Estimator uses the level of human chorionic growth hormone (hCG) in a woman’s body, which typically doubles every two to three days, to deduce when the pregnancy started.  “If you were to actually crack open a standard pregnancy test today, you would find one strip,” explains Clearblue Brand Manager Kirsten Suarez. “This new test has two strips inside: a high-sensitivity and a low-sensitivity strip that work together to measure the concentration of hCG.” Of course, you won’t crack open your pregnancy test. All you’ll see is the word “pregnant” and a week estimate: 1-2, 2-3, or 3+ weeks. A 2-for-$15 test can give you an immediate estimate of how far along into your pregnancy you are.  More from Prevention: The Key To IVF Success Clearblue says its product yielded 99% accuracy when detecting pregnancy and 93% accuracy when estimating weeks of pregnancy during clinical trials with more than 2,000 women. Couple that with the fact that its been sold in Europe for 5 years, and you’ve got a pretty compelling product.  Except for the fact that there are exceptions. The test can’t tell the difference between women who have high levels of hCG because they’ve had a bun in the oven for a while and those who are having multiples. It also can’t tell the difference between women whose hCG levels are low because they just got pregnant or because they’re having complications.  “The issue is, if a pregnancy is not going well, the hCG level doesn’t increase—failing pregnancies or ectopic pregnancies can give you lower values,” says Mary Jane Minkin, MD, a Prevention advisor and clinical professor of obstetrics, gynecology, and reproductive sciences at Yale University School of Medicine. “It’s a reasonable way to get an estimate, but there are so many variables.” Taking the test can’t hurt, as long as you approach the estimate with a healthy dose of skepticism. And no matter what your reading is, seek prenatal care and an ultrasound—it’s the only way to really get a grip on the status of your pregnancy.  More from Prevention: The IVF Approach With A 62% Success Rate