I am worried I might not be the father as my girlfriend was seeing another man within a week of being with me. I used a conception calculator online from the ultrasound due date "posted below" and it makes me confused. If anyone can provided me with any thoughts it would be greatly appreciated.
Ultrasound Date 11/19/2015
Ultrasound GA: 11 weeks 3 Days
Ultrasound EDC: 06/06/2015
Possible dates of conception: 8/29/15, 9/5/15.
First Day of Last Menstrual Period: August 31, 2015
Probable Date of Ovulation: September 14, 2015
Possible Dates of Conception: September 10 to September 18, 2015
Due Date: June 6, 2016 (40 weeks)
Based on a June 6 due date her last period started on August 31, 2015. She was not pregnant before that.
She would ovulate between September 11 and September 15, based on a 28 day cycle. The egg would be viable for up to about 48 hours after ovulation. Your sperm can remain viable for up to about 5 days - max.
That gives a window of from September 6 to about September 17 in which she could become pregnant. When she actually ovulated OR conceived no one can tell after the fact - not even her doctor.
Due dates are based on her last period. You add 40 weeks to her LMP start date. Ultrasound dates are estimates as well.
You don't say when you had sex with her but a DNA test is your BEST option at this point to determining who is the father. It can be done now, using a blood sample from your girlfriend and one from you. They can "pull" the babies DNA out of her blood. Google "non-invasive DNA testing." It is expensive.