Today’s pregnancy tests work by screening your urine or blood for hormone called human chorionic gonadotropin (hCG). This hormone is produced right after a fertilized egg attaches to the wall of the uterus, and then it takes about 48 to 96 hours for levels to be detectible in your urine.
However, fertilization itself can take as long as 3-4 days after intercourse, and then implantation can take 6-10 days after that. Also, sensitivity between tests can vary, from brand to brand and by individual test kit.
Test sensitivities are indicated in mIU/mL, the lowest amount of hCG in the urine that tests positive. So a test that can detect 15 mIU/mL will be twice as sensitive as one that detects 30 mIU/mL, and will be able to show a positive sooner. How much liquid you’ve had to drink before a test and how fresh the pregnancy test kit is can also affect your results.
So the important thing to know is that even if you use a good-quality test two weeks after you think you may have conceived, it’s still possible to get a negative test and be pregnant. When pregnancy tests say they’re “99% accurate” this refers to how accurate they are compared to other tests on the market, not how likely they are to give you a correct result when your period is late.
So if you get a negative test but still suspect you’re pregnant, test again in 24-48 hours, using your first urine of the morning. If your period is more than two weeks late, call your doctor and schedule an appointment. Other medical issues, such as thyroid problems, hormone imbalances and nutritional deficiencies can also cause skipped periods.
And you can also get a false positive on a test, if you’ve been taking certain fertility medications or have recently had a miscarriage.