Hi,
I used bio oil religiously from the beginning of my pregnancy and I gave birth to my 8lb 9oz daughter in September and I have no stretch marks on my belly or thighs so I would reccomend it, plus it smells lovley! It's not that cheap but a little goes a long way, i used it twice daily and used less than 2 bottles. Unfortunatley my boobs did not escape the stretchmarks but as they went from a b to an f cup over the pregnancy I don't think anything would have helped, lol! (and my gorgeous girl is worth every one)
Also a lot of it is down to genetics, if your mum and sisters got them then it's likley you will too. Most creams say if they're not suitable for use in pregnancy and lots of shops do specific pregnancy ranges.
Good luck and enjoy your pregnancy.