Cannabis users hoping to have a child should ease up on their smoking, researchers say - and not just because it's bad for any potential baby. The drug could be preventing them from getting pregnant in the first place.
Scientists at the Eunice Kennedy Shriver National Institute of Child Health and Human Development in the US looked at a study cataloguing the experiences of more than 1200 women who'd suffered a previous pregnancy loss. Each tried for up to six months to conceive, and filled out surveys on their drug use as well as undergo urine tests.
"While existing studies suggest that self-reported cannabis use is not associated with fecundability (the ability to conceive), self-report may not be reliable," the study, published in the journal Human Reproduction this week, says.
But by the end of the six-month study, 66 percent of those who didn't use cannabis had conceived, compared to just 42 percent of smokers - even though those who smoked regularly tended to have more sex.
"These findings highlight potential risks on fecundability among women attempting pregnancy with a history of pregnancy loss and the need for expanded evidence regarding the reproductive health effects of cannabis use in the current climate of increasing legalisation," the researchers said.
There was no link found between cannabis use and miscarriages in this particular study, suggesting rather than being harmful to any existing pregnancies, the cannabis was stopping conception from happening in the first place.
"Compared to non-users, cannabis users also had differences in reproductive hormones involved in ovulation," the institute said in a statement.
"These differences could potentially have influenced their likelihood of conception. Specifically, users had higher levels of luteinising hormone and a higher proportion of luteinising hormone to follicle-stimulating hormone.
"The authors also noted that animal studies had found that cannabis use could alter the lining of the uterus, making it less likely an embryo to implant and establish a pregnancy."
Cannabis use of the women's partners wasn't measured in the study either. Previous research has found a link between the father's use of cannabis and an increased chance of miscarriage, as well as lowered semen count.
"Until more information is available... women trying to become pregnant should be aware that cannabis could potentially affect their pregnancy chances."
New Zealand last year voted in a referendum to keep recreational cannabis use illegal, despite many medical experts pushing for its legalisation, saying it's less harmful than alcohol.