Hi Nelly,
My baby is due end of May, and I will prob have to go back to work mid Sept, when it is also still cold. However, just like you, I am not in the position to give up my career. I also would have wanted to get a nanny instead of a school, but the hours just wont work for me - I have to leave home at 6:45 and only get home just before 6pm, I dont think I can get a nanny working those kind of hours. I also cannot afford for a nanny to not show up for work or to be late, which often happens. So after thinking about it long and hard, I've decided to put baby in a creche that is down the road from my work. Yes, they do get sick at school, but I also think it helps them build immunity by being exposed to diff kids and their germs. I have friends who's babies stay at home, and they get sick anyway (and they are breastfed as well), so I don't think that keeping baby at home will guarentee them NOT getting sick.
Unfortunately in this day and age it is not possible for all moms to stay at home or to get nannies. School has benefits too, so I wouldn't upset myself about it too much if I were you. Good luck!