If it's not due to simply being too young, sometimes very skinny women simply don't have regular ones either. Something to do with very low % of fat in their body and their body says whoa you don't have enough reserves to have a baby to put it bluntly. Do I know of any scientific fact backing this? No but I personally know of several very athletic and lean women in their late 20's and early 30's that don't have a them often.