Stomach Stretch Marks HELP!

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I am an apple shape so I am holding most of my weight on my stomach and back. Around my belly button I have stretch marks, most old but i think more are comming through!
I am obviously trying to loose weight and I am doing exercise (running, 3DS and skipping) I am 5'2 and I weigh 140lbs.
Once i'm at my ideal weight will my skin be saggy and what can I do to prevent it? :(
Like I said, I'm young (18) and after I exercise I have a shower/exfoliate and then moisturise.
I am working on drinking lots of water too.