How bad is it to have sex on your back during the 5th month of pregnancy?

I'm 21 weeks pregnant, and today my hubby and I had missionary style sex because that was the most comfortable position for me. He didn't lay on my belly, but I worried afterwards because I read that you shouldn't lay on your back during sex after the first trimester of pregnancy. How true is this? Do you think this will cause any problems for me or my baby?|||I think as long as you are not on it for a long period of time you are ok. I've been doing it that way for the past month and I just prop a pillow under my side so that I am not completely on my back, just a little on it. So far everything is ok for me.|||As far as I know, sex during the second trimester isn't harmful to the baby, remember that the baby is surrounded by a lot of fluid and enclosed inside your uterus, pretty safe in there. Don't worry too much but check with your doctor if it'll put your mind at rest.|||My daughter and son are fine. I always laid on my back with my daughter,but with my son,not so much. They're both fine though. Just don't let him lay on your belly,which I'm sure he's not lol and doesn't want to.|||I have read somewhere that sex is completely safe "at any point in the pregnancy, until labor begins"|||id say it didnt affect your baby BUT i suggest not doing it anymore till the baby is born|||I would always lay on my back for sex when I was pregnant. My baby was fine.