Is using pot during pregnancy safe? Well, compared to what? We know alcohol, tobacco and other legal drugs can cause birth defects and a whole host of other problems. By contrast, a new study suggests cannabis use during pregnancy does not cause lasting effects on child’s brain development.

Researchers from the UK released new findings April 18 on using cannabis during pregnancy. The study investigated whether parental use of tobacco, alcohol and cannabis could cause lasting effects on the developing fetus’s brain. The study relied on a huge amount of data of 6,018 families from ALSPAC (a project to collect longitudinal health data on parents and their children) to look at the effects of maternal and paternal use of these substances during pregnancy.

While children of parents who had smoked tobacco or mothers who drank alcohol were more likely to have lower educational attainment than those who did not; parental cannabis use did not correlate with lower achievement. Still, researchers were careful to note that there were far fewer subjects in their study who used cannabis during pregnancy than those who used tobacco or alcohol. Further studies may reveal associations not found here.

This study is another piece in the controversial puzzle of whether cannabis use during pregnancy is safe. With legalization of cannabis, more women are choosing to use cannabis during pregnancy. I personally know several healthy babies whose mothers used cannabis throughout their pregnancy. Still experts seem to disagree about whether this practice is safe. Some claim it is dangerous for the unborn child; others suggest it could actually lead to better outcomes like improved scores on developmental tests.

The National Academy of Sciences and Engineering released findings on cannabis use during pregnancy back in January. They fully reviewed the pre-existing literature and found that there was substantial evidence that cannabis use was correlated with a lower birth weight but they were unable to link this with any negative later outcomes for the baby.

While concerns over cannabis’ effect on fetal development are high, it is difficult to point to any solid studies that show why. Maternal cannabis consumption is discouraged by the American College of Obstetricians and Gynecologists who advise that women pregnant or considering pregnancy should discontinue use of cannabis.

Still, even the ACOG points out that the research suggesting cannabis is harmful is very ambiguous. This is in part because there are so many confounding factors. Women who use cannabis during pregnancy are also statistically more likely have less education, have a lower weight during pregnancy, make less money, be in an abusive relationship and use other substances like alcohol, tobacco or cocaine during pregnancy.

It is difficult for researchers to separate these factors out and know whether trends like low birth weight are caused by cannabis use or are actually caused by something else. Still there are statistical methods able to sort through these confounding factors. The recent reviews and meta-analyses that applied such methods have found that when confounding factors are accounted for, lasting negative associations are no longer present.