First: Stretch marks occur from any sudden stretching of the skin. You can be 80 pounds, gain 10 additional pounds in a short time and get stretch marks. You can be 400 pounds and not have any stretch marks. What's the statistic-- 90% of all women have stretch marks? You might not even see them at the time.
Nothing makes them 'go away'. Once you have them, you're stuck with them, unless you have a plastic surgeon do a.. 'skin' transplant. o_o You can only make them fade by using vitamin E cream a few times a day. Tedious, they are. (Sorry for the OT. ^_^
For some reason, I think Britney already /has/ stretch marks. I think I've seen a photo or something of her dressed rather.. sloppily where they show.
Um. On to the pregnancy thing.
Honestly, I pity all children of big celebrities-- namely the ones who have parents with 'bad reps'. Honestly, can you imagine what a hard time they will have growing up? You think YOU got picked on badly at school-- imagine being the child of someone half of the nation can't stand and mocks regularly. And, already being chased by the paparazzi when only three months pre-natal? O_o;