Why do pregnant women smoke marijuana? Almost all women who smoke during pregnancy were smoking marijuana before conception. Many stop by the end of the first month, when they realize they are pregnant. However, about 15% of all pregnant women, most of them younger, smoke marijuana longer, because they believe that it is helpful for morning sickness. There is no evidence for its effectiveness for morning sickness, and all medical and public health authorities warn against its use. A few mothers continue throughout pregnancy, but most who used marijuana during second trimester stopped when morning sickness stopped. The mothers tell us that they believe cannabis from their known source is safer than a pharmaceutical from a large drug company, regardless of whether or not their doctor has recommended it.
Can marijuana harm the baby? Overall, the baby generally develops normally, unless the mother is a very heavy smoker. However, there is ample evidence that the development of the brain is adversely affected even by low levels of smoking, several times per week. We see no effects if mothers have stopped marijuana by 6 weeks of gestation, but effects are present if mother continues to use past the 6 week point even if they stop later in pregnancy. Their babies have poorer attention and cuddling than other babies. Cognitive effects have been recorded in Dutch studies throughout childhood, including increased Attention Deficit Disorder in school.
What about cannabidiol (CBD) or edible THC? Edible THC carries the same risks for the baby as smoked marijuana. Prenatal CBD has yet to be widely studied in pregnancy. Several early animal studies suggest that it may adversely effect the development of the testicles. 
Can choline or prenatal vitamins help? Higher levels of choline, which you can get from choline supplements, increase the development of the baby's brain to overcome at least some of the adverse effects of marijuana. During prenatal development, THC blocks the CB1 Cannabinoid receptor. The baby’s brain makes its own cannabinoids, which are needed for the development of nerve cells. THC blocks these natural cannabinoids and prevents their beneficial effects. A critical site is on interneurons in the cerebral cortex; interneurons are inhibitory switches that are critical to the brain’s ability to process information. The receptors that choline activates are on these same interneurons. Higher levels of choline compete with the adverse effects of THC. As a result, the baby’s attention and cuddling improve. Prenatal vitamins and folic acid do not counteract THC’s effect. These findings in early childhood do not rule out the possibility of later, as yet undetected effects of marijuana. We concur with public health authorities in warning against marijuana use in pregnancy, despite these initial findings of protective effects of choline. 
            Other drugs now being introduced into clinical practice and recreational use including ketamine, Methyl​enedioxy​methamphetamine (MDMA, also called Ecstacy) and psilocybin (magic mushrooms) have not been studied for safety in pregnancy or their effects on the fetus.
Read more: Interaction of maternal choline Levels and prenatal marijuana's effects on the offspring.
Prenatal choline, cannabis, and infection, and their association with offspring development of attention and social problems through 4 years of age