It depends on how many times you guys have had sex to determine that but go with you're first ultrasounds that should be the most accurate.
We only had sex that one time and soon after found out I was pregnant. My first ultra sound wasn't until I was about 5 months and she couldn't tell me an accurate due date because the babies genes take over and there's no way of telling?
Then I would count 38 weeks from the day you had sex since that was the likely time of conception. They count 40 weeks from your last period which includes 2 weeks before actual conception.
38 weeks from the time of intercourse would be closer to the right due date
Count 38 Weeks from the date you guys had sex would be my best answer
Either count 3 months back from the dat you conceived or 3 months back from last menstrual in my case it worked my due date is in July I counted 3 months back from Oct 23