Put the following fears aside and go for it. In spite of your increasing girth, it's going to be a lot easier to pull off now than after the baby arrives. Your fetus is so well protected in the amniotic sac that virtually none of the sexual gymnastics you two may be capable of will affect it. Nor will your partner's penis be able to reach the baby. Even if he were that well endowed, the cervix is tightly clamped shut.
Some women even experience their first orgasms during pregnancy.
The genitals are engorged, the nerve endings more sensitive, and oxytocin, the hormone that maintains a pregnancy, is known to ignite lust. There's no evidence that sex causes labor. Stimulation of the breasts does speed up the production of oxytocin, which can cause contractions when you're near term. But, as you will no doubt discover, you can experience lots of contractions without going into labor. Oral sex is out.