Are you pregnant or just asking? If you are pregnant, I don't think you want to wait around and "see" if you have the kind-of genes that get stretch marks. I'm assuming you are trying to avoid them at all costs!. I've had five children ,all were over 9 1/2 lbs... my first child was 11 lbs and my last 12 lbs. (Yes, he came out looking like he could walk already.. but I like them that way...they don't look like they'll break if you breathe on them and if they loose a little birth weight the first week noone is nervous.) ANYWAY... back to your question. I would (and did) start applying Shea Butter (pure and natural is the best... you can get the finest that I know of at cloudninedowntown.com for a good price) when you first realize you are pregnant... I probably did not do it too often at first.. maybe three times a week... and then when I started to show (with the first child at about 4 1/2 months, with my fifth child about the first week!!! Things change).. I was applying nightly (belly, thighs, breasts, butt, hips) and when I was huge (I can not even express how HUGE I was), I was applying ever time I was in the bathroom... and if you've ever had a 12lb baby sitting on your bladder then you know that was quite often. Well, it paid off... after all that trauma to my skin, I have very few stretch marks and the ones I did get are very, very faint ... and after having a C-section with the 12 pounder, the shea butter really worked wonders on the scar. I'm glad I kept my skin elastic and supple. I didn't want to wait and see what kind of genes I had.
Kookie is right though,... water is so important for your skin and the baby.