For the most part fertility docs, obgyns and midwives assume that unless a patient checks it too early, that urine pregnancy tests are pretty accurate. However, a new study calls that into question. Researchers at Washington University in St. Louis looked at urine tests commonly used in hospitals and found a high proportion of what we call "false negative" results, meaning the test was negative but the woman is still pregnant. The reasons have to do with the chemistry behind the tests. Like blood pregnancy tests, urine kits are designed to test for a hormone called hCG, or human chorionic gonadotropin. Without boring you with all the details, lets just say that hCG exists in many different forms and the types of hCG released can change during the course of the pregnancy. So, while these tests are very accurate early on around the missed period, the types of hCG a pregnant woman secretes may change a few weeks later making it harder to pick up on the urine test.