I think I would wait until late October or early November at least when it is a little cooler. I know with all 4 of my children I could have been ready to go physically in a month, but my concern would be the heat and possibility of sunburn....not so much the germs. I know their immune systems are fragile, but my doctor told me that infants actually have their mother's immune system in them until about 6 months of age. That is why after about that point, a lot of parents find their little ones with colds, that they just can't get rid of totally. They actually are building their own immune system until about 18 months of age.
My doctor always told me just to be a little more careful....I mean going to the grocery store, or attending any kind of community function, or even a sibling could bring it home from school, daycare, or gymnastics.
That being said, most of the sunblock says six months and older...my doctor's advice was he would rather have block on, then to burn. I think the worst part would be the heat, and the sun being so strong that if you were to take a baby out of a stroller, often of course, being that young, you have more of a risk of burn.
If you decide to go Good Luck, and just be extremely careful with the sun. Best wishes to a great pregnancy and a healthy little one when he/she comes.