Found a pretty relevant paper:

"Prenatal and breast milk morphine exposure following maternal intrathecal morphine treatment"--Journal of Human Lactation, May 2000, 16(2): 137-42

Mom was recieving intrathecal morphine during pregnancy due to RSD of right lower leg (10 year duration). Baby was born 3 weeks early, but did very well, with no signs of narcotic withdrawl. Mom's leg pain increased significantly at 29 weeks gestation, requiring 2X-3X the usual amount of morphine, but the pain went back down again by ~1-2 months after delivery.