Hi, I would go with the doctor's assessment. When was your first ultrasound done, and at that ultrasound (based on the size of the baby) did they tell you an estimated due date?
Hmm, if you knew the blood types and characteristics of both candidate fathers, that might (or might not) open up the possibility of reasonably safe testing during pregnancy (based on amniotic fluid).
The only certain test is genetic screening but the baby's not arrived so it's way too early to try for that. The doctor's view seems fair enough for now.