So, you enjoy the green stuff, but you're pregnant. First of all, congratulations! Secondly, you may be wondering whether can you still smoke weed and use cannabis products while pregnant. At this point, it appears that you'll be putting your unborn child at risk of serious developmental issues. This is because research does seem to indicate that marijuana and pregnancy do not go well together.
Medically, the jury is still out regarding the relative safety of tetrahydrocannabinol (THC), and pregnancy is a no-go area for rigorously designed scientific research. So, the investigation is mainly animal-based or done retrospectively with children whose mothers smoked marijuana during term.
Yet all is not as simple as it seems.
Damage to the unborn fetus of animals due to the mother's THC consumption is already a proven certainty. This is true of even low doses of THC, a study close to completion shows.
The study is being conducted at the Schulich School of Medicine and Dentistry, University of Western Ontario in Canada. Very low daily doses of THC were administered to pregnant rats throughout the course of the pregnancy until birth. These doses didn't affect the maternal rats in any way. (At high doses, THC causes the euphoric high marijuana is known for, as well as agitation, paranoia, delusions, etc. in some.)
Study leader Daniel Hardy, a professor at the school, explained that the small THC doses were designed to mimic what expectant human mothers may currently be taking to treat nausea, anxiety, and decreased appetite.
When the rat pups were born, researchers stopped with THC administration, so the babies were not exposed to any THC in the breast milk.
Here are the alarming results the researchers have discovered so far:
Because these effects were not self-correcting in the baby rats, the research is expected to go on till March this year.
But what happens to a human baby whose mother consumes THC while pregnant? Does the child grow up to live a normal life?
According to some research, the answer to this is also troubling.
First, a few notes about human research—pregnant mothers are not a group easily assessed, for obvious reasons. Because of this, well-designed, double-blind, placebo-controlled studies are impossible to perform.
Therefore, the available research was and is still done retrospectively, which carries the inherent risk of study design flaws. So, the research outcomes were conflicting, and they should not be seen as conclusive evidence about marijuana and pregnancy.
As our main source, Marijuana and Medicine: Assessing the Science Base, National Academies Press (U.S.) points out: 
"It is important to keep in mind that studies like this provide important data about the risks associated with marijuana use during pregnancy, but they do not establish the causes of any such association.”
Yet it is undeniable—the results of various studies were disconcerting, to say the least.
The cognitive functions of children whose mothers smoked marijuana during term were measured in what was called the Ottawa Prenatal Prospective Study, 1978. 
Pertaining to this:
"Children of mothers who smoked either moderately (one to six marijuana cigarettes per week) or heavily (more than six marijuana cigarettes per week) have been studied from the age of four days to 9–12 years ...
... The children in the different marijuana exposure groups showed no lasting differences in global measures of intelligence, such as language development, reading scores, and visual or perceptual tests. Moderate cognitive deficits were detectable among these children when they were four days old and again at four years, but the deficits were no longer apparent at five years.
Prenatal marijuana exposure was not, however, without lasting effect. At ages 5–6 years and 9–12 years, children in the same study who were prenatally exposed to [marijuana] smoke scored lower on tests of language skills and cognitive functioning."
The same source proceeds to quote other, more recent studies:
"In another study, 9 to 12 year olds who were exposed to marijuana prenatally scored lower than control subjects on tasks associated with ‘executive function,’ a term used by psychologists to describe a person's ability to plan, anticipate, and suppress behaviors that are incompatible with a current goal."
Again, the researchers hasten to point out that this data was collected from the mothers, who described their children's behavior. Anecdotal evidence is always a subjective, less reliable way to measure outcomes, as opposed to the more controlled, scientific method.
The authors also qualify that these results are not conclusive proof that marijuana use was necessarily the cause of cognitive problems.
"Mothers of the marijuana-exposed children were more likely to describe their offspring as hyperactive or impulsive than were mothers of control children ... The underlying causes might be the marijuana exposure or might be more closely related to the reasons underlying the mothers' use of marijuana during pregnancy.”
However, numerous animal studies still demonstrate that THC is detrimental to the cognitive development of the fetus, and later the offspring. THC and pregnancy definitely appear to be mutually exclusive. 
THC has many healthful uses, and it could very well be helpful for pregnant women in a specific preparation. However, all the available evidence at this point in time indicates that a cautionary approach would be best. Rather, don't smoke marijuana while pregnant. 
Now you may be asking yourself whether CBD is safe to use during pregnancy.
The honest answer is that we do not know yet. There is no clinical evidence to support or warn against the use of CBD during pregnancy.
According to anecdotes, it seems to be safer than THC-laden marijuana, but there's no way to tell for sure till more research has been conducted.
Marijuana and pregnancy don't seem to go well together at this point. Though one could assume CBD would be better than THC while pregnant, it would probably be best to remain cautious.