I've heard that August has a very high birth rate due to more people "getting together" around Christmastime, but I have no idea if it's true. You might check a birth registry or something if you're really curious - if people are having more sex in winter, then the birth rate will go up late in summer and early in fall. It might vary from place to place, as well.

I had read that a lot of 'birth explosions' occur nine months after a severe winter storm that results in trapped conditions.

i guess that makes sense, I prefer summer myself.