Not the only reason. Time of year also matters a lot for more "natural" reasons. IIRC mid-September is the peak and it goes down on either side, so late December is going to be pretty low anyways. Then you add in C-Sections and it drops to the absolute bottom.
Tangent but it occurred to me the other day that it takes 9 years before one experiences a birthday on every day of the week. Over the course of a lifetime maybe the dispersal evens out but it seems like some of us are just destined to have more birthdays on the weekend.