Avoiding the birth of a child right before winter would be advantageous for its survival… winter being (on average, not all regions/locales obviously) the hardest month to survive evolutionarily speaking this would be a beneficial trait and be likely to stick around. Ideally you want your early hominid babies at the end of winter through spring and summer so at worst they are though the first few fragile months that put the most drain on the mother, before the harshest winter weather sets in.

While not globally consistent due to regional weather differences it makes sense for there to be a broad average seasonal benefit.