Pregnant women of the world, please see a doctor AS SOON AS you think that you might be pregnant. Not only can you get a second pregnancy test, but you can start your prenatal care early. Fetuses need healthcare too, but the most important thing might be the first sonogram to estimate the age of the fetus. Combined with knowledge of your exact LMP (the first day of your last period), doctors can make better decisions about your healthcare...everything from deciding when you might need an induction to whether your fetus is growing normally.
I always wondered by doctors get very persnickety and anal about "dating" the pregnancy, but now realize that it is a fundamental and often underestimated part of the process.