December guy is the father. The ultrasounds are conclusive. You have short menstrual cycles compared to average, so don't worry if they tell you a number of "weeks pregnant" and it comes up a few days earlier than your last period, this would be to be expected. The number to pay attention to is the estimated due date. Your ultrasounds indicate conception in mid-December. None point to conception in early or mid November. If you had gotten pregnant that early, there is no way the ultrasound on January 1 would have failed to see an embryo wiggling and twirling around like a teddy bear, when it instead saw nothing. Even if you feel guilty about the event in November, that does not add up to the baby being from that regrettable incident.