If you don't know your LMP, they'll use the ultrasound date as your due date. Typically, my cycles are much longer than the average woman. So, it'll be 15 weeks since my LMP, but I'm only 13 weeks pregnant, because I ovulated a good two weeks later than the average. So, your ultrasound date may line up with you LMP, but it is more likely that it will line up with your date of conception. (Just take two weeks away, and that is the time frame of when the spermies met the egg.) When I go to the doctor, they have a made up LMP for me since my LMP and gestation don't ever match.
at 7:17 PM on Apr. 27, 2011