Yeah you'd have to consider optimal fertility cycles and mating/seasonal habits and tendencies. I remember looking at a birth rate chart for the months and we had a very cyclic pattern going, where there would be heavy births after colder months and an obvious 9 month barrier, that would decrease fertility, but obviously this was just the most common, we have pretty crazy reproductive options compared to some of the larger more social and intelligent mammals. Probably because we domesticated, which throws in a whole new set of pressures on birth cycles.