I'm going to base my reply on research where possible, but most of the pertinent studies on the effects of smoke on fetal development have been based on cigarette rather than marijuana research. Will it Hurt the Baby . Several studies have implicated regular use of the drug in the cause of birth defects, inadequate fetal growth, and depression of the nervous system in the newborn. The likelihood of other drugs or herbicides contaminating marijuana place the baby at even greater risk. Nevertheless, you should make your obstetrician fully aware of your use of the drug so appropriate tests can be performed to be sure the baby is all right.
Smoking releases gases that take the place of oxygen in the mother's blood.
We don't know how much oxygen reserve a fetus has, but it stands to reason that it has far less than an adult. To understand why this happens, think about an adult who smokes. There are substances in smoke that actually poison the cleaning system in the lungs, throat, sinuses, and ears of a person who smokes, and to a lesser degree, in those nearby who happen to breathe in the smoke. Babies, however, have very tiny noses and lung passages, so the same amount of mucus that mom has no trouble dealing with is an enormous amount for a newborn. They also don't know how to blow their noses, and they tend to breathe through their noses rather than their mouths.