As cannabis surges in popularity and more places legalise recreational weed, a growing number of people are using the drug in early pregnancy.

A large new study suggests there may be unexpected impacts of that choice. A cross-sectional analysis of 11, 489 children, 655 of whom were exposed to THC in the womb, has found cannabis use during pregnancy is tied to a small elevation in psychotic-like behaviours later in life.

These include aggression towards others, as well as attention and social problems.

While it could be that women with these behavioural features are more likely to use cannabis during pregnancy, the relationship stood even when other confounding factors, such as genetic predispositions, were considered.

Whether or not this link is causal is another matter - after all, there are many other factors the researchers may not have considered - but in the context of other research, it's an interesting link worthy of further exploration.

Animal models suggest THC - the psychoactive compound in cannabis - acts through endocannabinoid receptors in the developing brain, and these don't show up in the human fetus until five or six weeks into gestation.

Incidentally, this is when women generally learn they are pregnant. As such, the authors of the new study focused on people who continued to use cannabis even after finding out they were pregnant.

Children who were exposed prior to a mother's knowledge of pregnancy but not after showed no differences to those children that were not exposed at all. This further supports the idea that endocannabinoid receptors may play a role in child development.

Indeed, several other lines of evidence have shown prenatal cannabis exposure is associated with decreased attention span and some behavioural problems in children.

But while research on the health effects of cannabis is slowly catching up with legalisation, data on cannabis use during pregnancy is still lagging far behind.

And that could be inadvertently harming the next generation. A 2019 study of over 450,000 pregnant women found cannabis use more than doubled between 2002 and 2017, reaching 7 percent. It's now the most popular illicit drug for pregnant women to take.

Cannabis is reportedly used to deal with nausea and vomiting in early pregnancy, but there's little evidence to say whether this actually works or if it's safe.

Until we have those answers, researchers say health care professionals should discourage those who are pregnant, breastfeeding or even contemplating having kids from using cannabis.

Evidence suggests THC concentrations in the fetus can differ depending on whether the mother uses once per month or once per day.

But the dosage of weed during pregnancy and the timing of the exposure has not been studied very much at all. Nor have researchers properly examined the differences between ingesting edibles or burning flower.

As such, there is currently no known safe level of cannabis use during pregnancy or lactation.

The potential risks have led the American College of Obstetricians and Gynaecologists (ACOG) and the American Academy of Pediatrics to both advise against using cannabis in early pregnancy or while breastfeeding.

Even the US Surgeon General advises against cannabis use during pregnancy.

"THC has been found in breast milk for up to six days after the last recorded use," his announcement explains.

"It may affect the newborn's brain development and result in hyperactivity, poor cognitive function, and other long-term consequences."

That message clearly isn't getting through. Surveys show pregnant patients do not feel like their doctors are providing them with useful information about cannabis, and this has driven some to the internet for answers.

A study looking at a very small sample of pregnant women in Jamaica is one of the most shared pieces of information online. Conducted during the 90s, the positive results show no clear cause-and-effect relationship. But these results are based on outdated cannabis that was not nearly as high in THC as many plants are today.

In the years since this research came out, there have been several larger studies that have found much more worrying results. A study in Canada found that among more than 660,000 pregnancies, cannabis use was tied to a greater risk of preterm birth, placental abruption, greater admission to the intensive care unit, and lower weight and Apgar scores.

Some of these adverse effects appeared to be dose dependent. For instance, women using cannabis at least weekly, compared to less often, had a roughly 5 percent greater chance of preterm delivery.

Another study in the US found prenatal cannabis use was associated with a 50 percent increased likelihood of low birth weight.

While alcohol and tobacco during pregnancy are also linked to adverse health outcomes, these are already well documented. But many women don't know these are risks that might also come with prenatal exposure to weed.

"Therefore," reads an analysis from ACOG, "because of concerns regarding impaired neurodevelopment, as well as maternal and fetal exposure to the adverse effects of smoking, women who are pregnant or contemplating pregnancy should be encouraged to discontinue marijuana use."

The study was published in JAMA Psychiatry.