In quantitative hCG blood tests, the amount of water you drink should not affect the results. The makeup of your blood does not change based on your fluid intake; normal hCG levels from a blood test should not fluctuate in accuracy by time of day, by how much water you drank or how much food you ate.
However, drinking a lot of water might affect the accuracy of a urine-based home pregnancy test -– very diluted urine could theoretically result in a light-colored test result line on a home pregnancy test.
In very early pregnancy, depending on the sensitivity of the individual pregnancy test, drinking a lot of water and taking the test later in the day could theoretically affect whether or not a line shows up at all in a urine-based pregnancy test. But this should only be true in roughly the first week after conception.
Source:
American Pregnancy Association, "Human Chorionic Gonadotropin (hCG): The Pregnancy Hormone." July 2007. Accessed 17 Jan 2008.
