I started using it as soon as I found out about 19 weeks now and no stretchmarks so far
I would start using it now
With my daughter I didn't get stretch marks until after I delivered and they were on my backside
I started as soon as 5 weeks. At least 2 times a day. 21W now and no stretch marks but i am still little
Use them the end of first trimester and only use approved creams or oils. Vitamin E oil is one of the best things for stretch marks and scars (research it) or a lot of women use cocoa butter with a bit of collagen to help with elasticity.
I use bio oil nd it says 2 start using it in the 2nd trimester i dont know bout other ones