Was telling my pregnant wife the other day, it doesn't make sense to mark your months on gregorian calendar. There is no reason for your body to tell whether the month is 30 or 31 days. There is no basis other than what a pope decided centuries ago. Probably that's why they go by weeks anyway.
The moon, on the other hand, has a pretty stable orbit and goes around in a very calculable way. It averages lunar months lengths in a very stable way.