Yes you can lift weights. I do and it helps a lot. I do then for my flabby triceps lol. I Also do squats as the shower is running before I get in to help my thighs and butt. Prenatal yoga too for flexibility. You can get a cheap DVD from Walmart.
Go swimming! Even if you walk laps it feels so good to feel weightless. I'm 23 weeks with twins and measuring 39 weeks and I just started swimming last week. I was basically just lying on the couch for 5 months, even after a week I feel so much better...like I'll actually be strong enough to have these babies! Give it a shot, it's so worth it.