Another theory is like the humps (sic) of camels this subspecies of womanosaur will be able to survive off the stored sustensnce until the next season!
Actually you are on to something there. About 8 pounds that a woman gains while pregnant are there as a nursing reserve. If a woman does not nurse her baby for at least a month or two, she will have a much harder time loosing this weight. If females were not very mobile for a few days after giving birth, something the evolution of a larger head might have influenced, then having this reserve to live off of and produce milk from had a distinct survival advantage.