After the babies are born they crawl into Mom’s pouch and swallow a nipple.  They drip feed instead of nursing, as the nipple is long like a piece of spaghetti.  Baby possums don’t know how to nurse like our mammals do.  If Mom is hit by a car and killed, her milk secretes a toxin that will kill the babies.  You can open the pouch and remove the babies by gently and slowly pulling them off the nipple so as to not do damage to their throats and mouths.  When the babies are old enough to leave the pouch they cling to the mother’s back. 

They fall of and she doesn’t know it and she does not come back and collect her babies.  They are capable of feeding themselves at a very young age but as they are small they are easy prey.  If you see a possum under 6” long not including the tail, it should go to a wildlife rehabilitator.  They do have sharp teeth so be careful when handling them.