My Son Was Told He May Be A Father Due

My son was told he may be a father due to Ultrasound scans showing the date of conception being at the same time my son was with the girl. She had been with a different guy 16 days earlier but we all went with the ultrasound. The baby was born 3 weeks early at 5 pound which complicated things. The baby has similarities to my son and not the other guy. We have recently done a dna which came back negative but there were three delays and they said that they found female dna that they were having trouble extracting a profile. we never sent a female sample, also the case number wasnt the same for my son as for the baby, so we are very unsure. My question is if three scans estimate a due date the same is it possible for those scans to be out by 3 weeks?