Marijuana Use in Pregnancy By the Numbers

Marijuana is the most commonly used illicit drug in pregnancy. An informative and well-researched review in the December 2015 Gray Journal by Dr. Metz and Dr. Stickrath of the Denver Health Medical Center (in Colorado, where, just to note, marijuana is legal) takes a long look at what we know and don't know about the effects of this substance in pregnancy. Here, I've broken it all down by the numbers.

The number of individuals, both pregnant and not, using marijuana has increased drastically. In 2007, only 4.8% of people in the US reported using marijuana in the last month. In 2013, this number shot up to 7.5%.

As has been publicized heavily in the media, four states have legalized recreational marijuana- Washington, Oregon, Colorado, and Alaska, and 23 have legalized medical marijuana. 

Perhaps equally as interesting as the increased use of marijuana is the fact that the marijuana used today has a much higher concentration of the active ingredient, THC, than the marijuana in the 1980s. This is an important point for understand the research around marijuana and pregnancy. Many of the studies draw on data from pregnancies in the 1980s, when the THC concentration was much lower than it is today.

So how many pregnant women are really using marijuana in pregnancy? It's hard to know, and the estimates range from 3 to 30%, depending on who you ask and how you ask.

For instance, if you only ask women who were already using marijuana before pregnancy, you'll get a much higher number who report using marijuana in pregnancy- 60%.

To muddy the waters even more, there is a pretty poor overlap between women's report of using marijuana and whether they have a positive urine screening test for THC. Some women will have a positive urine test and report that they don't use marijuana- perhaps because they are afraid to share this information or because the test was a false positive. Other women report using marijuana but come back with a negative urine test, likely because the amount they use is less than would be detected on the urine screen or they haven't used it recently enough to show up on the test. So- if you are a researcher, who do you count as a "marijuana user" in your study? By report? By urine test? By frequency?

With all of these complications in even defining who to use in the study, it is no wonder that the existing research is somewhat sparse and, when it exists, the studies conflict. For instance, some proponents have proposed that marijuana can help tame nausea and vomiting in pregnancy. In the two studies conducted, one showed it made nausea worse and the other showed some women thought it helped. For now, the true effect of marijuana on nausea and vomiting in pregnancy is unknown.

The most pressing questions surrounding marijuana in pregnancy revolve around whether using marijuana in pregnancy is harmful to mother or infants. A few studies suggest that infants born to mothers who use marijuana during pregnancy are smaller- around 80 to 100grams smaller at birth. We don't know yet if this small difference in weight makes an actual difference for clinical outcomes.

The most concerning data available is about the increased risk of preterm birth and stillbirth associated with marijuana use in pregnancy. One study showed a 2.34 times increased risk of stillbirth for infants with positive THC in the umbilical cord at birth. A number of studies show no increased risk of preterm birth, but other show an increase of 1.3 to 1.5 times. For all of these studies, yet another factor that makes the results even harder to interpret is the confounding of other variables- namely use of other substances like tobacco or other illicit drugs. For instance, if a pregnant women uses tobacco and marijuana in pregnancy, is the infant's low birth weight or increased risk of preterm birth due to the tobacco or the marijuana? It's hard to pull apart these confounding variables in large population studies.

In yet another very difficult realm of research, scientists have tried to figure out if exposure to marijuana during pregnancy will affect children's later development. Marijuana is known to  reduce the number of dopamine receptors in the emotions center of the brain of developing fetuses. However, now imagine doing tests on 10 year olds. Are their answers the result of being exposed to marijuana in pregnancy or to all the different things that happened to them in those 10 years of life.  Again, the studies vary with some showing no differences between children exposed and not exposed to marijuana in pregnancy. Other studies tend to show lower scores in visual processing, impulse control, attention, and memory associated with exposure to marijuana in utero.

One of the most common questions new mothers have is whether substances - everything from vitamins to alcohol to, yes, marijuana- is safe to use while breast feeding. In the case of marijuana and all illicit drugs, The American Academy of Pediatrics recommends against breast feeding when women are using marijuana or any illicit substance. Marijuana does pass freely into breast milk to the infant.

Some hospitals have posed the question: since breastfeeding is contra-indicated if using marijuana, should they withdraw breastfeeding support to mothers who report using marijuana? Since the benefits of breastfeeding are well known, some providers think it's best to teach the skills necessary and also advise their patients to stop using marijuana while breastfeeding in the hopes that the new mother will be successful in both breast feeding and stopping marijuana. 

If this blog post goes to show anything, it's that we have a lot left to learn when it comes to this controversial and complicated topic.