you just never know how things will turn out.
i had never had real stretch marks with any of my other pregnancies until my twins (even though I gained the same amount as I did with one of my singletons). that being said, the marks are not all that bad, in my opinion. they faded fast and are noticeable to the point only where I would not wear a bikini, but i am not the bikini type anyways.
i went back to ballet when the twins were around 18 mo, and firmed right up, but before then, my tummy was a bit poochy. i was never able to get back into shape without getting some good exercise, so for me that was key.
before my current pregnancy (I am 23 weeks), i was pretty darned pleased. yeah, sure, when i am parallel to the ground, there is some extra skin that falls a little lower than the rest of my tummy, and yes, my breasts do not look like they did when I was 20, but hey, i have to say, that change is worth having two little twin boys who think I am the most wonderful woman on earth!
when they say, "you pretty mama!", I have to soak it in and take it.
I do remember being concerned when I was preg with them and feeling similar to you, and feeling that was HAS to be normal, as I know so many other mothers who went through that (even with singleton pregs). Try not to stress too much - that first year, you will be too busy to think much about it anyways, and after that you will be in awe at how amazing your body really is. Really. I'll be praying for you and those little ones. PEace!