Marijuana persists in your body for over a month. Moreover, whatever you ingested, smoked or injected after the 20th week of pregnancy will show up in the baby's meconium (first stool), which can be tested without your consent. Not only is the action of smoking itself is detrimental to the health of the baby, but we do not know how cannabis intake can affect the developing brain. You want to eat right and stay away from all unneeded drugs and alcohol while you are pregnant and nursing or mothering young children. Please take good care of yourself and that way you can be sure you are taking the best care of your child!