How did your due date change from before to after your first prenatal appt?

A🕊 • 34. Married to DH for 9 years. #1 born 2016. #2 born 2018. Fertility issues - Tried 3 IUIs. Army wife. LDS.
We do our own calculation as soon as we get a BFP that is based on the first day of our last period, then we go to the doc and they measure baby and give us a more accurate date. How different were those two due date calculations for you?