For the new study, researchers in the Netherlands followed more than 7,000 pregnant women, 3% of whom acknowledged smoking marijuana at least during early pregnancy. They found that babies born to marijuana users tended to weigh less and have smaller heads than other infants.
What's more, the study found, the longer a woman had used marijuana during pregnancy, the stronger the impact on birth size - suggesting that the drug itself was to blame.
And while most marijuana users in the study also smoked cigarettes, the drug appeared to have effects over and above those of tobacco. In fact, marijuana showed stronger effects on birth size than tobacco, the investigators report in the Journal of the American Academy of Child and Adolescent Psychiatry.
The findings suggest that marijuana use, even restricted to early pregnancy, may have irreversible effects on fetal growth, write the researchers, led by Hannan El Marroun of Erasmus University Medical Center in Rotterdam.
To prevent this, they add, women who smoke marijuana should quit before becoming pregnant.
The study included almost 7,500 pregnant women who were surveyed on their use of alcohol, tobacco and drugs, and had ultrasounds to chart foetal growth during the first, second and third trimesters.
Overall, 214 women said they had used marijuana before and during early pregnancy; 81% quit after learning they were pregnant, but 41 women continued to smoke marijuana throughout pregnancy.
The researchers found that, on average, marijuana users gave birth to smaller babies, particularly those who had used throughout pregnancy.
Women who had smoked only during early pregnancy had babies who were
156 grams - about 5.5 ounces - lighter than infants born to women who had not used the drug. Women who had continued to smoke past early pregnancy had babies who were 277 grams, or nearly 10 ounces, smaller.
Based on ultrasound, marijuana use only in early pregnancy impaired foetal growth by about 11 grams per week, while use throughout pregnancy slowed foetal growth by roughly 14 grams per week. That compared with a deficit of 4 grams per week with tobacco use, the researchers found.
Similar patterns were seen when the researchers looked at foetal head circumference.
According to El Marroun's team, mothers' marijuana use could stunt foetal growth for several reasons. Like tobacco smoking, it may deprive the foetus of oxygen. It is also possible that the by products of marijuana directly affect the developing nervous and hormonal systems of the foetus.
Finally, the researchers note, pregnant women who use marijuana may have other factors in their lives - such as a less-than-healthy diet or chronic stress - that could contribute to poor foetal growth.
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