Although the risks of smoking tobacco while pregnant are well documented and well publicized to the general population, women continue to smoke cigarettes during pregnancy. Smoking cigarettes is known to increase a woman’s risk of having a low-birth weight baby and increases the risk of preterm delivery.  While clinicians often ask about and discourage tobacco use during pregnancy, much less attention has been devoted to the use of marijuana during pregnancy, despite the fact that marijuana is the most commonly consumed illicit drug among pregnant women.

Early studies suggested associations between intrauterine cannabis exposure and various neurodevelopmental and behavioral problems in the exposed offspring: increased tremors in newborns, decreased mental scores in infants, and lower intelligence scores at age 6.  While some studies have demonstrated intrauterine growth restriction in cannabis-exposed infants, this has not been a consistent finding.

In a recent study, Marroun and colleagues examined the relation between maternal cannabis use and fetal growth in a population-based sample.  In order to address some of the limitations of previous studies, the researchers examined a subset of women enrolled in the Generation R Study, a multiethnic population-based prospective cohort study designed to identify early environmental and genetic determinants of growth, development.  7,452 mothers with singleton pregnancies were enrolled during pregnancy and provided information on substance use and fetal growth.   Substance use was measured via self-report and timing and frequency of use were obtained both before and during pregnancy and included illicit substances including cannabis, as well as tobacco and alcohol.

Fetal growth was determined using ultrasound measures in early, mid-, and late pregnancy. Additionally, birth weight was assessed.

The statistical model used to analyze the data adjusted for gestational age, maternal age, body mass index, height, education level, national origin, maternal alcohol use, parity, gravidity, fetal sex, and maternal psychopathology (as determined by the Brief Symptom Inventory).

The results showed that of the pregnant women included (n=7,452), 245 women (3.3%) used cannabis only before pregnancy, and 214 women (2.9%) used cannabis before and during early pregnancy.  Of the 214 women, 173 (81%) quit using cannabis in early pregnancy, whereas 41 women (19%) continued using throughout pregnancy.

Of the women using cannabis during pregnancy, 85% also smoked tobacco during pregnancy.  The number of daily cigarettes did not differ significantly between women who smoked tobacco and used cannabis versus women who only smoked tobacco.   In total, 1,453 mothers (19.5%) smoked tobacco during pregnancy (but used no cannabis).

The results indicated that using cannabis before pregnancy did not affect fetal growth.  However, maternal cannabis use during pregnancy was associated with growth restriction in mid-and late pregnancy and with lower birth weight.  Growth restriction was most pronounced for those with continued use throughout pregnancy.  Fetal weight in cannabis-exposed fetuses showed a growth reduction of -14.44 g/week and head circumference was reduced by -0.21 mm/week, compared to non-exposed fetuses.

The authors concluded that maternal cannabis use, even over a short period of time, can negatively affect fetal growth.  Furthermore, cannabis use during pregnancy resulted in more pronounced growth restriction than maternal tobacco use.

The authors reported several limitations to this study including that the data relied on self-report and therefore may underestimate use.  Also, the effects of tobacco use were controlled for but because tobacco and cannabis were often used at the same time, the authors were not able to ascertain information based on cannabis use alone.

While there are limitations to the current research, many studies have indicated negative outcomes associated with intrauterine cannabis exposure.  The authors recommend that standard care should include screening for cannabis use and educating mothers about the consequences of prenatal maternal cannabis use.

April Hirschberg, MD

Marroun HE, Tiemeier H, Steegers EA, Jaddoe VW, Hofman A, Verhulst FC, Brink W, Huizink A. Intrauterine Cannabis Exposure Affects Fetal Growth Trajectories: The Generation R StudyJ Am Acad Child Adolesc Psychiatry, 2009;48(12):1173-1181.

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