The doctors are saying I conceived feb. 11th, I was just wondering how far along am I? One doctor that I seen is saying I'm 19 weeks and the other is saying I'm 21 weeks I don't know what to believe.

Its on a bell curve. There are 2 basic ways of telling how far along you are. One is by measuring you and seeing how far along you are. The other is by ultrasound and measuring the baby. Either way it's a bit of a guess. There is no to the exact date way of knowing.

They are likely going with your gestational age, which would be from your last period and your fertilizational age, which is when you likely conceived. There is a window of time between the two, this is also why due dates are not exact and you have several weeks at the end for a "normal" time to give birth. Unless you had medical assistance to conceive there is no way to know exactly when you conceived, only to get close, as califdadof3 said. Ultrasound and baby measurements will give you the most likely but even this is not exact, especially the farther into the pregnancy you get.