I should take her collar off before she gives birth, if I were you. You don't want kittens getting caught in it. If you can, try to avoid letting her out after she has the kittens (but do put her collar on if she does go back out) - well meaning people might see a friendly cat who is obviously feeding kittens and decide she needs to be "rescued", especially if it's obvious that a collar has been recently removed. Or perhaps get one of those name barrels and write a message inside saying "I live at ... please let me go back to my kittens!"