It's been 41 days since my last period started. I have taken two home pregnancy tests, both positive. I then had a test at the doctor's office, which was negative. I have since had two more positive home tests. I am feeling pregnant and have another test scheduled at a different doctor's office. Whatever the result, how I could have had four positive results at home and a negative at the doctor's?

--Lynn

The current generation of home pregnancy tests is quite accurate -- 99 percent if used correctly. The tests detect hCG, the pregnancy hormone, and most today detect a specific subunit of hCG, thus lowering the chance of a false positive due to an interaction with other hormones. The sensitivity of the tests can vary, however, such that they may require a higher level of hCG before the result will be positive. Urinary pregnancy tests done in a doctor's office are no more or no less accurate than most home tests.

One reason for a conflict between the tests at home and in the doctor's office may be how concentrated your urine was at the time. If you used first morning urine at home, after not having gone to the bathroom for several hours, then the concentration of hCG in that specimen would have been higher; if you emptied your bladder before going to the doctor and then gave another specimen, there might not have been a high enough concentration of hCG in that specimen. Most urine pregnancy tests instruct you not to urinate for two to four hours before performing the test. A second reason for the discrepancy is the one I mentioned above -- your home test may detect a lower level of hCG than the doctor's test.

The most accurate testing for pregnancy involves drawing blood. I suggest you have a blood test done to determine things once and for all. Blood tests can detect extremely small amounts of hCG, much lower than any urinary test.