At first they base is off of the first day of your last period but after the ultrasound they base it off of how far along the baby is measuring, my first date was 11-15-16 and then I got 10-13-16 and now my due date is 10-23-16 so he probably changed it off of babies measurements :-)
It's normal, especially if you didn't know your conception date. Due dates are based off of babys size and development but some women have large/small babies so it's sort of a guesstimate. Your best bet is to go by the due date given from the ultrasound because that's always nearly exact (due to it being based on development of the baby). But you could still go over or earlier than that
Both were based on a ultrasound
There are two ways to calculate a due date. One is by ultrasound- these involve your babies measurements and are most accurate between 10-16 weeks since babies should be measuring the same despite genes and maternal health. The second is by your last menstrual cycle- assuming that you have a regular 28 day cycle and that you ovulated on the 14th day of that cycle the doctor would have counted 40 weeks from then.
My best guess is that your first due date was calculated the second way, and then after the ultrasound your due dates were calculated with measurements. I would go with your ultrasound due date during the 10-16 weeks window.
Actually the later ultrasounds aren't as accurate as the early dating ultrasounds...they say to always go by your earliest ultrasound...
Brit, when was your first ultrasound performed? I agree with maw, that's almost always the most reliable one.
It's curious that they changed the due date with a later ultrasound, instead of recognizing that your baby is large for gestational age.
I agree with others, if the first was an early ultrasound (first trimester) that is far more accurate way to date the pregnancy than a later ultrasound. I think you need to discuss this with your Dr, your baby may justice be tall.