conception date

I know it’s almost impossible to know exactly which day you conceived if you were having sex multiple times around your fertile / ovulation window, but i was just wondering, what’s a more reliable way of knowing your due date? because my periods are a bit irregular so i don’t know they’re the most reliable way of knowing your exact due date, but if we were going to go based off that my due date would be march 15th, but my first ultrasound at 8 weeks said the baby was measuring to be due march 13th. so i was just wondering if one is more accurate than the other? 😊 knowing this would help me also know which day i conceived