When you are expecting do the dr. Usually go by your last day of your last cycle or your first day of your last cycle?

First day. Weeks of pregnancy (weeks of gestation) are counted from the first day of the last cycle, and that day is called lmp. If the LMP is unknown, but an ultrasound is able to tell how many weeks after conception the fetus is, then the LMP is estimated to be 2 weeks before the ultrasound-determined conception date.