A large systematic review of prospective and retrospective cohort studies found little or no effect of maternal marijuana use on birth weight, stillbirths, preterm births, or congenital anomalies (TABLE1-8). Some studies found lower birth weights and some found higher birth weights. The authors couldn’t perform a meta-analysis because of heterogeneity, but estimated a clinically insignificant difference of 100 g. Most studies were limited by failure to account for concurrent maternal tobacco smoking.
Moreover, all studies used interview data to determine maternal prenatal marijuana use, which can be subject to large recall bias. A multicenter prospective study of 585 pregnant women that compared interview data with serum screening to identify tetrahydrocannabinol (THC) found poor correlation between history and laboratory validation, for example.1 Only 31% of pregnant women with positive THC testing self-reported marijuana use (31% sensitivity), and only 43% of women who reported marijuana use had a positive THC screen (43% specificity). Most studies didn’t quantify marijuana use well and didn’t associate use with trimester of exposure.
The authors also point out that marijuana potency has increased substantially since the 1980s when many of the studies were done (THC content was 3.2% in 1983 and 13% in 2008); prenatal marijuana use in the present day may expose the fetus to larger amounts of THC.1
A 2016 retrospective cohort study of 56 mothers who reported prenatal marijuana use found no differences in preterm birth, low birth weight, or Apgar scores.2
Neurodevelopmental effects on infants, long-term effects on children, teens
Three prospective cohort studies evaluated neurodevelopmental outcomes in neonates and infants, and 2 studies continued to follow children into adolescence.1,3 All found essentially no differences associated with prenatal marijuana at birth, throughout infancy, and through age 3 years. The studies had the same limitations as those described previously (potential recall bias for identifying which children were exposed to marijuana prenatally and poorly quantified marijuana use not well-associated with trimester of exposure).
The Ottawa Prenatal Prospective Study (OPPS) examined 140 low-risk pregnancies in white women of higher socioeconomic status who used marijuana during pregnancy.1,3-7 Investigators considered: socioeconomic status, standard demographics, obstetric history, and use of other drugs, tobacco, and alcohol. Using a standardized newborn assessment scale, they found subtle behavioral differences at one week but not 9 days. Investigators evaluated children again at 3 years of age, school entry (5 or 6 years), and 9 to 12 years.