Excercise, 18 weeks

Ka

So my doctor told me not to lift more than 20 pounds. Well my husband and I recently joined the gym again and so far I've walked on the treadmill 1+ miles, and sometimes go on the bike. I just wonder if there's anything else I can do to give birth easier. I plan to have a natural birth, no meds, but I am quite scared of tearing, etc. Are there any other excercises I can do that could help me give birth naturally and safe for me and baby?