My Partner And I Are Having A Child. We Found

My partner and I are having a child. We found out on June 23rd that she was 4-5 weeks pregnant. On July 22nd I had my first ultrasound at the hospital. The ultrasound lady told me I measured at 8 weeks 5 days. The doctor on call that night had my hcg quantative level tested and it was 170171. He told me I was 8 weeks 6 days pregnant. Well July 30th at my first obgyn appointment my doctor told the ultrasound tech to do a very quick ultrasound and give her a rough estimate of how far along I was. Less than 4 minutes doing an ultrasound the lady said around 9 weeks 2 days.
On June 11th my partner and I had thrown a party where we ended up getting the law involved because a buddy of his claimed to have spiked the punch and sleep with me. My partner and I just want to make sure this is our child and not his buddies.