You're not supposed to lie on your back too much during the second and third trimesters, so straight-on missionary is pretty much off the menu. But if that's your favorite position, turn it on its side by putting a pillow under one side of your butt and back. Have him lie facing you on his side and intertwine his legs with yours. If your belly's getting in the way, he can lean his torso away from you and thrust more straight up into you. All the benefits of missionary — that is, you can just lie there (during pregnancy this will seem hugely appealing) — without the notable drawback of compressing the vena cava, a vein you'll be needing to properly Give Life and whatnot.
2. Womb with a view
Pregnancy means lots of doggy-style sex. Spice it up by moving to the living room and kneeling on the couch, facing the back. He'll also be in fine position to reach around and stroke your labia, which — like seemingly everything else on your body during these months — is swollen as hell. In this case though, that's a good thing. Real good. Oh, you'll see.
3. Peas in a pod
Spooning is a good go-to position during the third trimester because your tremendous belly will be getting in the way of your Love (this will not be the last time your kid does this). To make it mind-blowing: hold a bullet vibrator on your clit by squeezing your legs tightly together as he rocks into you from behind. No better way to feel both turned on and well-loved than a cuddly spoon fuck.
4. Bedside service
Prop yourself up on a ton of pillows and sit at the edge of the bed, opening your legs wide. He can kneel or stand and adjust your pillow situation for height. Once you hit optimal pillow placement, rub your clit while you both watch his smooth, slow thrusts into your gorgeous round body. And if you can't reach your clit, have him rub it for you.
5. The mama bear moan
There's a lot about the whole "birthing a child" experience that will make you feel animalistic and undignified (oh, just wait 'til you meet the breast pump). So embrace your primal nature by having him kneel on the bed and sitting back on his lap in sort of a kneeling reverse cowgirl — then do it like they do on the Discovery Channel (sorry, that song was gross, but you get it). Moan loudly and let yourself go feral. Cathartic and super hot.
I think that even when you're not pregnant, these sex positions are worth a try, because why not? Just remember to always play it safe!