February through April are waaaaaay OP.
For example, spring babies are more likely to inherit heart diseases and die younger, but less liable to suffer mental problems. Fall babies are apparently the least fortunate. They're most likely to develop not only neurological conditions like ADHD, but also respiratory illnesses and viral infections, possibly due to increased rates of infections during winter pregnancies, according to researchers. However, these birth months benefit from lower incidences of cardiac disease, so as a fall baby, there's no reason you can't live a long, demented life.
Babies born during the summer months appear to be healthiest, as they avoid the block of heart diseases associated with early springtime births, as well as the neurological quagmire of the fall months. Scientists believe it's partially due to extra vitamin D from increased sun exposure, but the ebb and flow of more than 150 different bodily chemicals can contribute to disease.
Random chance it is!
There's a minor tradeoff, as women born during the summer months are slightly less fertile, but that's only fair. Otherwise, with numbers on their side, those summer super-babies would surely take over the world.