MzMorgan,
Your due date indicates that you ovulated/conceived around Jan 5. Sex between Jan 1 and Jan 6 got you pregnant. Your current BF is the daddy.
If you want to count back from the gestational age, you first need to determine where they measured it from; ovulation (38 week pregnancy) or ovulation minus 14 days.
Working from your due date does not have this problem as it is the same for both 38 week and 40 week schedules.
Take care