The effect of marijuana use on pregnant women and the children they rear is a controversial subject. It is also a minimally researched subject because conducting studies that expose women to pot is considered unethical. There are ways around that, however; looking at historical and retrospective data has allowed researchers to learn a lot about how the substance affects both pregnancy and birth.

A new study published in the Journal of Perinatology found that marijuana use "does not increase the risk of adverse obstetrical outcomes or fetal anomalies." Though it does appear to increase the risk of having a baby that is smaller than it should be for its gestational age, researchers at the University of Cincinnati determined that using cannabis does not carry the same long-term risks associated with using other drugs, including tobacco, during pregnancy.

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Here's the thing: it is probably not in the best interest of women or their baby to smoke pot during pregnancy. Still, mothers have been doing it for thousands of years, finding that it is more effective (and safer) at treating morning sickness and labor pains than prescription medication, for example. There is plenty of evidence supporting the medical benefits of pot for chronic pain patients—but there is simply not enough research on marijuana and pregnancy to say if it is a better option for pregnant women.

What we do know

There are earlier studies showing that marijuana use during pregnancy does not result in miscarriage or birth defects. An ethnographic study of newborns in Jamaica—24 of whom were exposed to pot before birth, and 20 of whom were not—found no evidence linking mothers' marijuana use to birth abnormalities. And that is also what Dr. Carri Warshak, co-author of the most recent study, found.

"I was surprised to see no increase in adverse obstetrical outcomes such as preterm delivery, preeclampsia (a blood pressure disorder of pregnancy), and even stillbirth," Warshak told ATTN:. "Tobacco increases the risk of stillbirth, as does obesity and advanced maternal age. However marijuana use did not show even a trend toward an effect on the clinical outcome."

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That said, Warshak also expressed concerns over the increased rate of neonatal intensive care unit (NICU) admissions for children whose mothers reported using marijuana. "When moms used marijuana, there was a 54 percent increased risk of needed to admit the baby to the NICU for various reasons," Warshak added.

That's a problem, as is the fact that children who were exposed to marijuana were smaller, on average, than those who were not. I asked Warshak what that finding meant, and she emphasized that "size is more than just a number."

"Smaller babies have higher rates of NICU admission, supportive care, feeding and jaundice issues, [and] prolonged hospitalization," she said. "In addition, we are only beginning to understand 'prenatal programming' where stressors during pregnancy (that often lead to poor fetal growth) lead to downstream health implications, including higher rates of hypertension, obesity, [and] diabetes in the offspring of these complicated pregnancies."

Of course, the University of Cincinnati study also relied on retrospective data, collected from more than 6,400 mothers who had their children at the school's medical center between 2008 and 2011, and approximately 58 percent of marijuana users included in the study were concurrent tobacco users, so it is difficult to determine whether or not these findings offer a comprehensive overview of the relationship between cannabis and pregnancy.

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Here's how other drugs affect pregnancy.

Tobacco: Increased risk of miscarriage, placental abruption, low birth weight, Sudden Infant Death Syndrome (SIDS), according to the Centers for Disease Control and Prevention.

Cocaine: Heavy cocaine use during pregnancy is associated with higher risk of preterm birth, miscarriage, placental abruption, and birth defects.

Heroin: Heroin use during pregnancy is associated with increased risk of neonatal abstinence syndrome (NAS), which causes a child to become dependent on the substance.

Methamphetamine: Regular meth use during pregnancy is associated with decreased oxygen flow to the fetus, inhibited growth, and increased risk of placental tearing and premature delivery.