How can I manage stretch marks while I'm pregnant?

Not much. It is largely a genetic thing. Some women get them and some do not. They are caused by rapid expansion of skin.
Worth a try. Can't guarantee it as stretch marks are related to genetics, hormones, and how fast your skin expands but it is worth trying bio-oil. You can get it otc for under $10. Also, watch your weight gain - about 25 lbs your entire pregnancy. You really only need a few hundred extra calories a day and much more nutrition so pick healthy, colorful, organic foods and not junk.