My doc said no more sex because I have a short cervix and I've already been having contractions. With my last pregnancy I had a lot of pre term labor and sex did cause it once and I had to get a shot to stop it
I think it only works when you’re body is ready. I was already 3cm and my cervix was softening. Had sex one night got contractions all throughout the night and my water broke the next morning. I was 38 weeks
I'm not sure there's statistics on that but it shouldn't put you into early labor if your pregnancy is healthy. Labor is actually started by hormones secreted by the baby. While sex can help the process it doesn't induce.