I don't think marijuana will effect a pregnancy test result, alcohol may, as it goes into the urine. If you are pregnant it would be wise to stop smoking marijuana now, as it may have terrible effects on your baby, including low birth weight, early labour onset and withdrawal type symptoms, not forgetting that it will most likely cause learning difficulties in older childhood. Smoking tabacco, marijuana and the consumption of alcohol are all going to have negative effects on an unborn baby. Smoking tobacco and marijuana around bubby even after birth is thought to be a possible cause of sids (sudden infant death syndrome).