I don't think I would be keen to go, but it is a great opportunity for you! Personally, I would prefer the April hol (but I don't have a prob flying in the 2nd tri), but it sounds like that's not going to be happening now.

I know you're not keen to tell people, but if you are going to agree on going to the US I think you should tell your ILs anyway. I'm sure it won't make a difference to them, but it is their hol too, and it will be easier for everyone to plan around the fact that you will have a little baby with you ie. accomodation etc. Much as they will love having a grandchild it might not be their idea of a holiday being woken up through the night IYKWIM!

Good luck with your decision!