Yes, orgasms CAN (can being the operative word!!!) trigger labor. The body releases endorphins during an orgasm, which can send the body into such a relaxed state that, as you can imagine, the uterus relax and then starts to contract. More commonly, O's cause the contractions that can lead to full blown labor. However, it's not all that common and unless you have a history of preterm labor or a weak uterus/cervix, I wouldn't put the love making on hold at all!!!!!!!!
Additionally, some chemical/hormone in the sperm can also trigger contractions that lead to labor. later on in your pregnancy, it's likely that you will experience braxton hicks contractions, but again, not all contractions lead to labor so unless you are already at high risk for one reason or another (cervical length, weak uterus, history of preterm labor, miscarriage for unknown reason...) keep on going at it.
For me, with my very high risk pregnancy, i continued to have orgasms until about 21 weeks, when my doctor said that because of my size (nearing full term) that my uterus didn't need any encouragement to relax as it was already stretched out and that at that point, it was risky to continue with such activities. In a non high risk pregnancy though, it's rarely discouraged.