(CNN)More and more women are using weed in pregnancy but they may want to think twice.
Researchers have found a link between marijuana use by expectant mothers and autism and childhood psychosis. Now, a small study has shown how cannabis use can affect the placenta and may be linked to higher levels of anxiety, aggression and hyperactivity in children.
The US researchers looked at 322 mother-child pairs based in New York City who were part of a wider research project on stress in pregnancy. When the children were between 3 and 6 years old, hormone levels were measured from hair samples, electrocardiogram recordings were used to measure heart function, and behavioral and emotional functioning was assessed based on parental surveys.
The study team also looked at placental tissue collected at the time of birth for some of the participants.
"This new study supports a growing body of evidence that smoking cannabis during pregnancy is associated with adverse outcomes for women and their children," Dr. Daghni Rajasingam, consultant obstetrician and spokesperson for the Royal College of Obstetricians and Gynaecologists in the United Kingdom, told the Science Media Centre in London.
"We know from previous studies that using cannabis during pregnancy is linked to impaired fetal brain development, stillbirth, low birth weight, and pre-term birth. This new evidence adds to these existing safety concerns, suggesting that cannabis use in pregnancy could lead to higher anxiety, aggression, hyperactivity, and levels of the stress hormone cortisol in the children," she said.
"There is only a small sample of women and children used in this study, and we would like to see more research done in this area."
Negative impact on kids' mental health
The children of mothers who used cannabis during pregnancy showed higher anxiety, aggression, hyperactivity and levels of the stress hormone cortisol, compared with children of mothers who did not use cannabis during pregnancy, the study found.
The analysis of the placental tissue, which involved sequencing RNA -- molecules similar to DNA that are part of the genetic code -- revealed that maternal cannabis use was associated with lower expression of immune-activating genes, including cytokines, which are involved in protecting against pathogens.
"This is a well-designed study, with good methodology, and laboratory design with follow up of children. It is interesting that they were able to look at placenta signalling and link those findings with childhood outcomes," said Dr. Darine El-Chaâr, a maternal fetal medicine specialist and clinical investigator at Ottawa Hospital Research Institute in Canada, who was not involved in the research.