Taking any form of medication during pregnancy remains a contentious topic at best, no less because of the inherent problems posed by testing on live human subjects. The same applies to cannabidiol (CBD) oil and pregnancy. No sane mother would want to subject her unborn child to untested medicine, knowing there’s a chance it could affect the baby adversely.
For this reason, most research appears to be done in vitro or on animals. Also, the majority of this literature seems to be focused on prenatal consumption of tetrahydrocannabinol (THC) or marijuana, with no conclusive data about cannabidiol (CBD) and pregnancy.
Let’s look at what is available.
For the uninitiated, a quick review to clear up terminology, etc.
Cannabidiol (CBD) and tetrahydrocannabinol (THC) are two of over 100 cannabinoids found in the Cannabis sativa plant. They are the most well-researched for their healthful properties, with indications for treatment of the following and many more:
These cannabis compounds engage with the body’s own endocannabinoid system (ECS), which is “comprised of endogenous cannabinoids, the metabolic enzymes responsible for the formation and degradation of endocannabinoids, and the cannabinoid receptors and their interacting proteins.”
The ECS, found throughout the body in various sites, is involved in many physiological processes, and several therapeutic effects have been ascribed to phytocannabinoid activation of this wondrous system within the human body. 
Yet, CBD and THC have diverse functions and actions in the body. The most notable difference between them is THC’s psychoactive effect (euphoria and relaxation, mostly) versus CBD’s fairly neutral effect on mental functioning and awareness.
In this, THC is singled out as the possible culprit for causing neurodevelopmental problems in unborn children.
What does the research say regarding CBD oil and pregnancy? Let’s have a look.
Well, this is easy, because a rather extensive online search reveals that there seems to be currently no clinical data available regarding CBD oil and pregnancy.
A rigorous study is needed to establish short-term and long-term consequences of CBD ingestion during pregnancy, especially in terms of safety.
Upon further scrutiny, it seems the general scientific consensus is that ingesting or smoking marijuana in large amounts could be especially harmful to your baby’s neurological development, but research remains uncertain.
Looking at the available scientific material, it seems that when you are pregnant, it would be prudent not to use marijuana recreationally or medicinally in large quantities, if at all. For mothers-to-be, many study outcomes suggest that “erring on the side of caution” is probably the safer route to take here.
In a well-researched article called Lasting impacts of prenatal cannabis exposure and the role of endogenous cannabinoids in the developing brain, published in Future Neurology, 2011, Chia-Shan Wu and colleagues soberingly conclude that “marijuana abuse during pregnancy and adolescence represents a major health problem owing to its potential consequences on neural development. Prenatally cannabis-exposed children display cognitive deficits, suggesting that maternal consumption has interfered with the proper maturation of the brain.” 
Some online sources are lauding marijuana as a solution for pregnancy-related nausea, but another, more recent article, The Risks of Marijuana Use During Pregnancy, 2017, warns:
“Pregnant women and those considering becoming pregnant should be advised to avoid using marijuana or other cannabinoids either recreationally or to treat their nausea.”
According to the authors, there is furthermore “substantial theoretical justification” that marijuana interferes with the neurodevelopment of the fetus, and they list the following side effects of high-THC weed use before birth:
Other material links prenatal marijuana use to late fetal and perinatal death too. This is a sobering picture indeed, but not all well-designed research says the same.
In a New Zealand study, Maternal use of cannabis and pregnancy outcome, published in the BJOG International Journal of Obstetrics and Gynaecology, 2003, over 12,000 pregnant women were examined. They were all enrolled in the Avon Longitudinal Study of Pregnancy and Childhood, and five percent of these mothers reported smoking marijuana before and/or during pregnancy.
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The authors concluded: “The results of this study suggest that the use of cannabis during pregnancy was not associated with increased risk of perinatal mortality or morbidity in this sample. However, frequent and regular use of cannabis throughout pregnancy may be associated with small but statistically detectable decrements in birth weight.” 
Also, in another laboratory-based study, Cannabinoids and the human uterus during pregnancy, the researchers note: “Both endogenous and exogenous cannabinoids exert a potent and direct relaxant effect on human pregnant myometrium, which is mediated through the CB(1) receptor. This highlights a possible role for endogenous cannabinoids during human parturition and pregnancy. These results also support the view that the use of exogenous cannabinoids during pregnancy is not linked independently with preterm labor.” 
This study seems to suggest that the body’s own cannabinoids (endogenous) play an important role in pregnancy. It remains to be seen if taking phytocannabinoids (plant-based) during pregnancy is harmful or beneficial.
So, research seems contradictory and inconclusive, which could be due to many factors. For instance, the article on neurology mentioned earlier uses the phrase “marijuana abuse,” which infers frequent ingestion of copious THC-heavy strains. This is not the same behavioral pattern observed in the large study of British pregnant women.
At this point, we can only speculate, though, and as mentioned, much more study is needed. In the meantime, the U.S. National Institute on Drug Abuse recommends that pregnant women not use marijuana at all, or only in very small amounts. CBD oil for pregnancy is not sanctioned either, for the same lack of evidence that it is safe for the fetus. Especially if you’re pregnant, never take any medicine or supplement without your doctor’s consent.