Wow! Well, I went at 18 weeks, and it wasn't so bad, just a little slower than normal, and it wasn't our first trip. I probably could have done it at 35 weeks this time, but by 37/38 I was miserable and there would have been no way! I would imagine that my Dr's would have said no way, too! Hopefully your little ones are still taking naps so you'd get a break every day. We went at the beginning of September, and the heat didn't seem to bother me, even being pregnant, but we always go August/September. We did go in March once, and it seemed just as hot!
Oh, and we're not even 20 minutes outside of DC without traffic. I had an interview once there that was mid morning, after rush hour, and I was surprised how close DC really is, LOL. Since traffic is usually present, I'd have to say it takes an hour or more to get where we are going, normally!
Is it just the three of you? We are so up in the air about #4, because of the age gap, and giving this one a playmate. Luckily, I just turned 30 so we have a couple years to decide, but I don't want to wait 5 more years or it kinda defeats the purpose of having another one for a playmate!