Ensure that you stay hydrated. It would be smart if you didn't stay in the sun more than half an hour at a time becuase you don't want to risk overheating. Get up go in the shade for a few minutes or jump in the pool and then get back to laying out. Ashelen had some good points that I won't repeat.
Unless you're feeling weak and overheated the baby should be fine as far as temperature goes....and the amount of sun that's safe for her is exactly equal to what's safe for you. As long as you don't burn (i.e. reapply your sunblock frequently, wear a hat and/or a cover-up when lying out) you should be fine. make sure to check your skin often to check for burns, reapply sunblock, and [sorry JUST got some bad news while I was typing this reply so my brain just completely farted].
just avoid sunburn, pay attention to make sure you don't feel overheated, drink lots of water and keep that sunblock coated you should be FINE. the vitamin D and sunshine are great for the baby and you :). and the swimming feels GREAT I spent my entire third trimester in the pool with Kahlan because it made me feel so much less uncomfortable.