Pregnancy can change how you feel about sex. I'm not talking about how some aspects of pregnancy like, say, morning sickness can put you off sex because you never want to become pregnant again. No, I'm talking about something entirely different.
Pregnancy brings with it hormonal fluctuations that can snap your sexual desire up and down like a yoyo. At some stages, you may be too tired and too nauseous to feel any sexual desire at all. Then at other stages you may be astonished by your powerful sexual appetite.
Sometimes people worry that sexual intercourse might be hazardous for the developing fetus. But the fetus is safely buffered by the amniotic fluid, and by the mucous plug blocking your cervix.Sexual intercourse poses no danger to your little one.
"Orgasms can cause uterine contractions, but these contractions are different from the contractions you'll feel during labor. If you have a normal pregnancy, orgasms — with or without intercourse — don't seem to increase the risk of premature labor or premature birth. Likewise, sex isn't likely to trigger labor even as your due date approaches."
http://www.mayoclinic.com/health/sex-during-pregnancy/HO00140/rss=1