I had a one time sexual partner on sept 19 and had my period sept 20-27. again oct 5-8. i met my boyfriend sept 28th. i'm currently pregnant and ultrasound showed 5 weeks 4 days on nov 18. could there be a possibility sept 19 guy is the father?

Pregnancy: If the ultrasound is correct, you are 39 days pregnant as of 11/18, meaning you conceived on 10/11. Menstrual period finished 9/27, so you ovulated approximately 14 days later (10/11). Fertile window is 4 days +/- ovulation day, so September 19th encounter seems to be out of the picture based on the information provided. All the best...