SO doesn't believe me! 😭😭

Okay so I'm not proud of this.. But before I was in a committed relationship I went out partying one time in my life.. I had protected intercourse the 7th of October. 
I am in a relationship sense the 24th and had a period on the 29th and continued having intercourse with my SO. The ultrasound dated me exactly with my LMP & conception dates. 
No way it could be a whole month off?!