Baby measuring behind?

[Leightonelizabeth]

Based off my LMP (12-13-22) , I’m almost 18 weeks pregnant. I went for an anatomy ultrasound today, and found out my baby was measuring at 16 weeks and 3 days. Which is more accurate to go by, my LMP or the ultrasound scan? I know for a fact when my period was, and my periods are pretty regular! This has thrown me for a loop because my due date could change by two whole weeks. I had one ultrasound in my first trimester (which I heard were more accurate), but doctor couldn’t get the measurements then. I am not understanding, if all babies are different and grow differently, why are we going off a second trimester ultrasound instead of LMP when my cycles always been regular.

LMP is a rough estimate based on a 28 day cycle. It doesn't take in to account anyone who has a longer cycle, ovulates late, or how long implantation can take (up to 10 days). Your scan will always be more accurate. Babies don't grow on a graph, you could be 2 weeks behind today and a week ahead next week.

Due date changing by 2 weeks is quite standard, if you had known at 12 weeks then it probably wouldn't have been such a shock.