I'd agree they are hereditary, I'm preg with my 3rd & the heaviest of my family (my sisters are blessed with my Mum's side of the family jeans)
Mum never got a stretch mark, neither sisters have stretch marks & other than my thighs verry thin & feint (Overweight more than preg!) I dont have any stretch marks!!!!

I put moisturiser on when I remembered which was most nights after my bath/shower & it was just any old moisturiser that was there at the time!!!