Many women rely on home pregnancy tests to give them their first chance to find out if they're expecting. And while the accuracy is high, there are some factors to factor in.
"Pregnancy tests detect HCG - it is a hormone produced by the developing placenta once an embryo emplants in the uterus." And Dr. Jana Allison OB/GYN says you can measure HCG in the urine - the method used in home pregnancy tests - or in the blood - the test favored at the doctor's office. "They do become positive earlier because there's more circulating HCG in the blood than in the urine."
Home tests claims to be 99% accurate by the time you miss your period. That margin of error depends on a couple factors. Adds Dr. Allison, "Some women will ovulate late or will implant late in the cycle, around the time they miss their period. So for them that test is not going to be 99% sensitive. So if you can wait to one week after your missed period, then it does become more sensitive for you."
So accuracy will depend on each patient's ovulation schedule. Dr. Allison says that can lead to misleading results. "False negatives are almost entirely because it's taken too early. If you get a negative test and you've missed your period, you need to repeat it in three days to a week. If it's still negative, most likely you're not pregnant. For a false positive, it would be for women who maybe have infertility and got an HCG injection or part of the HCG diet and have exogenous HCG and then it could be higher."