Hi, I'm pregnant in 13w1d. I wonder why? Bcoz we have sex in August 30 and sept. 1 and my last LMP was july 3rd week. Why my baby is 13Weeks and 1day?

LMP July 3rd, or 3rd week in July? Your post is confusing on that part.

If you had an early ultrasound, your pregnancy was likely dated from what was seen. This will be more accurate than dating from the last period. Being 13w1d would make conception around the end of August/early September time.