They can tell by giving you a blood test it will give most accurate due date by your hormone levels
I Will Tell You My Situation And See If It Makes sense. So The First Day Of My Cycle WAs October 20. I Didn't Ovulate Till November 2 Though So My Conception Wasn't Until November 2 But The Way They Date A Pregnancy Is The First day Of Your Last Period Or Cycle. So That Put My Due Date At July 25. Even Tho She Was 2 Weeks Late... I Know For A Fact That My Due Dater Wasn't Off Because I Was Tracking My Period. So For You When Did Your Period Start ? In February? Even Tho Your Conception Date Is March They Always Date It To Your Period Starting Even Though You obviously Weren't Pregnant Then. Its Because everyone Has A Different Cycle And doesn't Always Know Exactly When They Ovulated. Hope That Makes Sense. Sorry Fir All The Caps. My Phone Automatically Does It