ACOG

Obstetrician–gynecologists should be discouraged from prescribing or suggesting the use of marijuana for medicinal purposes during preconception, pregnancy, and lactation. Pregnant women or women contemplating pregnancy should be encouraged to discontinue use of marijuana for medicinal purposes in favor of an alternative therapy for which there are better pregnancy-specific safety data.

There are insufficient data to evaluate the effects of marijuana use on infants during lactation and breastfeeding, and in the absence of such data, marijuana use is discouraged.

Notably, 34–60% of marijuana users continue use during pregnancy, with many women believing that it is relatively safe to use during pregnancy and less expensive than tobacco 

It is difficult to be certain about the specific effects of marijuana on pregnancy and the developing fetus, in part because those who use it often use other drugs as well, including tobacco, alcohol, or illicit drugs, and in part because of other potential confounding exposures. Marijuana smoke contains many of the same respiratory disease-causing and carcinogenic toxins as tobacco smoke, often in concentrations several times greater than in tobacco smoke.

Studies of marijuana exposure during pregnancy are potentially subject to reporting and recall bias, often relying on self-reported habits, including frequency, timing, and amount of marijuana use. 

women using marijuana at least weekly during pregnancy were significantly more likely to give birth to a newborn less than 2,500 

 Several studies noted statistically significantly smaller birth lengths and head circumferences as well as lower birth weights among exposed offspring