As more people use cannabis for recreational purposes, attitudes towards the drug have changed. For example, research has shown that dispensaries often recommend cannabis—also referred to as marijuana—to pregnant women to ease pregnancy symptoms, especially morning sickness.

There is a growing body of literature attesting to poor child outcomes if cannabinoids are consumed during pregnancy. The exact effects on the developing fetus, however, remain unclear. Researchers in the US have now examined how timing of cannabis exposure during pregnancy impacts fetal development.

"We show that even when marijuana use occurred only in the first trimester of pregnancy, birth weight was significant reduced, by more than 150g on average," said Dr. Beth Bailey, professor and director of population health research at Central Michigan University and senior author of the study published in Frontiers in Pediatrics. "If that use continued into the second trimester, newborn head circumference was significantly decreased as well."

Continued exposure results in largest deficiencies

"These findings are important as newborn size is one of the strongest predictors of later child health and development," added Dr. Phoebe Dodge, the study's first author, a recent graduate of the Central Michigan University College of Medicine and an incoming pediatric resident at University Hospitals Rainbow Babies and Children's Hospital in Cleveland.

Recent work, including the research by Dodge et al., has shown significant effects of cannabis use on newborn size. "Size deficits were largest among newborns exposed to marijuana throughout gestation," Bailey explained. The babies born after continued in-utero exposure were nearly 200g lighter, and their head circumference was nearly 1cm less than that of babies who had not been exposed. Pregnancy cannabis use did not significantly predict newborn length in this study.

The effects the scientists observed have also shed light on patterns of use. Their study showed that occasional use, such as for first trimester morning sickness, may reduce fetal growth in the same way as continued use throughout pregnancy. The same is true for other use in early stages, including cases when someone uses cannabis not knowing they are pregnant.

Quitting before pregnancy is best recommendation

The authors pointed out that in their study they did not have information about how much or how often participants used cannabis. Their results were based on whether people did or did not use it at certain times in pregnancy. Therefore, the study could not establish if there was a connection between heavy use and more pronounced outcomes in newborn growth.

More studies are needed to determine whether timing or amount of use is most important when it comes to effects on newborn size, they wrote.

"The best recommendation is that women should be advised to quit marijuana use prior to becoming pregnant," Dodge said. However, quitting as soon as possible after getting pregnant is the second-best option to avoid long term adverse health and developmental outcomes. "There are some benefits of quitting among those who begin pregnancy using marijuana," she continued.