The smoke from Marijuana (much like cigarette smoke) crosses the placenta and goes straight to the the fetus, this means that any marijuana toxins that enter the mothers body enters the babies body. Like cigarette smoking marijuana can disrupt the oxygen supply and nutrient supply to the baby. This can restrict the development of your baby (including overall length, foot length, head size and body weight), and in rare cases premature birth, miscarriage and stillbirth.) Some women who smoke marijuana during pregnancy are more likely to give birth to babies with lower birth weight, delayed breathing, an increase in features similar to those found in fetal alcohol syndrome, tremors, poor eye sight, and a "hole in the heart". There's a study that shows that babies who were exposed to marijuana have a greater risk of developing asthma, chest infections, and other breathing problems such as wheezing.