Does Birth Control affect Pregnancy Test Results?
The pregnancy tests you buy over the counter are not affected by glucose levels in the urine, by urinary tract infections, or by being on birth control. They strictly measure the levels of HCG in the urine and are usually very sensitive when it comes to measuring the levels of HCG. These tests can pick up on HCG levels just after implantation of the embryo in the uterine wall. Then the HCG from the embryo is leached into the maternal serum and excreted in the urine in high enough amounts to showing up as a positive pregnancy test. The tests are so sensitive that you can tell if you are pregnant before your missed period.
How to deal with False Negative Test Results
Even though these tests are very sensitive for HCG, there can be false negative test results if you test before implantation has occurred or if the urine is too dilute. If you believe you are pregnant, but the pregnancy test is negative, repeat the test on a first morning specimen a day or two after the first negative test. The HCG levels double in 48 hours so that, if the test is negative at first, it will likely be positive in 48 hours as long as you are really pregnant. Most pregnancy test kits come in sets of two to three test kits per package so you have the opportunity to retest without having to purchase more kits. This is perfect for ultra-sensitive tests that may be negative if you test six days before your missed period but will be positive on testing again 48 hours later.
Once you have a positive pregnancy test, you do not need to retest because it will continue to be positive throughout most of the rest of your pregnancy. HCG levels peak at 8-10 weeks gestation and gradually go down after that.