I've had 3 miscarriages and I didn't abstain from sex in the first trimester of this pregnancy, I'm only 10wks.
I bled during weeks 5-6 with this pregnancy, only bright red one time, and then it turned brown for 2wks, and we still had intercourse during.
As for your question, I think it's orgasm cause orgasms involver a uterus contraction.
I had my ultrasound of course I was so excited to see my liitle one I forgot to ask the Dr my question! Lol oh well hubby and I agreed no sex until next appointment at least. I'm very happy to share with you all I got to see the little tyke and its heart is beating! I feel like I can relax now even though I'm only 6 weeks according to measurements today, I certainly have a good feeling on this one. :)