I had DC#1 at 18 and surprisingly I was not all that concerned about my body changing. Pre children I was a tiny size 6 and very petite at 5ft0. Naively I just thought I would snap back into shape again after the birth, and many people told me this (ha!). My idea was tummy gets bigger - baby comes out (somehow!) - tummy goes down to normal! Was not prepared for it being a full-body experience, with even my nose getting wider!!!
Despite just eating normally (and not even much due to sickness) I gained 3 and a half stone
and my bump was sooo big that on the ward the midwives kept assuming I was carrying twins - people were actually shocked when I said there was only one baby
.
After DC#1 I did not snap back into shape. It took 9 months for me to get to a size 8 (half a stone heavier than previously and up one cup size).
I have ALOT of deep stretchmarks from that pregnancy. However I would say to anyone who has them for the first time that they DO fade to skin colour. And your womb will take time to go down and tummy muscles will tighten. I found it helped to try not to put too much pressure on yourself - enjoy your new baby, and the rule - 9 months up, 9 months down. It also helps immensly if you have a partner who is still "into" you sexually during pregnancy and post birth.
My pregnancy with DC#2 I was not quite as big but still pretty impressive bump!
The legacy of the 2nd pregnancy has been very wrinkly tummy skin (think my tummy has been stretched to it's limit!), c-section overhang (had 2 c-sections) and slightly 'fallen' boobs. Again it took me 9 months-1 year to get to a size 10 (another half a stone heavier) and up another cup size.
I did relish being able to sleep on my tummy - actually jumped onto the bed tummy-first! , have sex in missionary again and feel like my body was my own again.
However pregnancy is temporary and a normal thing, it's only 9 months out of your life.
I put on half a stone after each baby which I find hard to shift (I dieted after DC#1 to no avail) - I'd practically have to starve myself lose that stone - and f**k that! I see this body as the body I am meant to have and actually prefer being this weight, and don't really mind my tummy sticking out a bit and being a bit flabby (well not enough to do sit ups and crunches!).
Once in my previous workplace ladie's changing room there was a conversation and a 'pulling up of tops showing of stretchmarks'. Compared to mine the other ladies stretchmarks were minor, so I showed mine and some of the other ladies recoiled in shock and gasped - seriously! I was a bit narked at the time but now I am strangely proud of that moment! 
Bodies never look the same after childbirth - it's normal, but not neccesarily "bad" . Anyone (usually celeb mags or twatty men) who say otherwise should be slapped over the head 
Sorry for the huge ramble, this issue is one of my passionate topics! There is a site/blog called "the shape of a mother" will is quite good.
Good Luck with your pregnancies
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xxx