I was on percocet thru out my pregnancy w/ my 2nd son. I was also concerned about him going thru w/ds after he was born. Most of the info I looked up online back then said it depended on how much u take and how close to birth its in your system. I believe I hadn't take any for about 2 days bfore my son was born & he did not seem to go thru any w/ds @ all. Hope this helps. Try googling the subject and see what u can find.
Being as it is in your 3rd trimester it is less likely to have an affect on the baby HOWEVER there is always a chance that it could. I believe percocet is considered relatively safe after the first trimester. Your baby may or may not go thru withdrawals but depending on the amount of medication you take it probably won't be serious. There's really no way to know for certain what the outcome will be until you have the baby.
It depends on how long you have been on it and if it is a narcotic.