Is it possible to conceive Dec 16th and be due October 1st

Patient: Is it possible for my boyfriends ex girlfriend to be due October 1st +/- a week if her last period was December 4th and she was 13 weeks as of march 29th? and the the last day of sexual contact with my boyfriend without protection was December 16th and she slept with someone else new years with protection?

Symptoms: Pregnant