In recent years, it has become clear that cannabis use during pregnancy can lead to unwanted developmental problems in a fetus. But now scientists believe that a father’s cannabis use can also be detrimental to a baby’s developmental health. Researchers have found that regular cannabis use can cause epigenetic changes to sperm that can have negative effects for developing babies.

A 2018 study published in the journal Schizophrenia Research found that a father’s secondhand smoke from cannabis consumption during a partner’s pregnancy was associated with more psychotic-like experiences for offspring compared to babies of fathers who did not use cannabis during this time.

The causality behind this association is unclear as secondhand cannabis smoke has been shown to induce little to no psychoactive effects in those who are in the same room as a smoking cannabis user but aren’t directly consuming the drug. A 2015 Journal of Analytical Toxicology study found that a nonsmoker would have to be confined in a small sealed chamber with no ventilation while a smoker continuously smoked flower for an hour to test positive for THC in a urine test.

The mystery behind the association may have been solved if researchers had been able to identify which participants had used cannabis before conception and examine the quality of their sperm.

A 2020 study published in the journal Toxicological Sciences found that male rats injected with THC daily for a month produced offspring with brain developmental abnormalities. The offspring suffered from slowed development and permanent damage to neural pathways that are involved in learning, memory, reward and mood.

The next year, the head researcher of the 2020 study, Theodore Slotkin, published a paper in the same journal that found that the offspring of male rats exposed to cannabis smoke showed signs of impaired dopamine pathways, which would make them more susceptible to chemical dependency in their adult years.

Slotkin has suggested that the developmental abnormalities discovered in the rats’ offspring could be connected to persistent changes in gene methylation found in rats exposed to THC that alters genetic expression of sperm cells.

It’s still too early to tell if the same sort of effects are present in the offspring of human cannabis users without more clinical trials, but the results are still very disturbing.

A 2018 study out of Duke University Medical Center published in the journal Epigenetics also found that cannabis use affected the genetic profile in sperm resulting in nonviable embryos. “In the absence of a larger, definitive study, the best advice would be to assume these changes are going to be there,” said the study’s lead author Susan K. Murphy “We don’t know whether they are going to be permanent. I would say, as a precaution, stop using cannabis for at least six months before trying to conceive.”