My last period was 12/10 & i had unprotected sex 12/18 where he finished inside me. i had unprotected sex again 12/28 & took plan b that day. baby is due 9/17 a & my first ultrasound was 1/24 measuring 5w6d based on info which resulted in pregnancy?

The only worthwhile answer & one that will hold up in court proceedings will be a DNA based paternity test. The NIPP is a non-invasive test performed on your blood as early as 8 weeks. They find fetal DNA in your blood & can test it for a variety of things including paternity. Ultrasounds and projected dates can give data, but a surprise at delivery is not worth a wrong call.

The best guess is 12/28 and that and your ultrasound suggest a slightly later due date around 9/20 (which says little about when the baby will actually come!). If the baby were to come significantly earlier than that and not appear premature, then 12/18 would be more likely. DNA testing would be required if you need more certainty.