How far does an umbilical cord drug test go back?

As long as it is in : Blood. , it will also show up in the baby's cord blood. How long that is depends upon the particular substance involved. You will have to look that up online yourself. You should obviously not endangering your child's health by using illicit substances during pregnancy. Please get the help you need to stop doing that unless it happens to be an opiate to which case you need to seek specialized counseling at a clinic that specializes in that. Best wishes.