Research suggests that marijuana use, a controversial habit in the US, may be damaging men's sperm and doubling their partners' risk of miscarriage. Researchers from Boston University surveyed over 1,400 heterosexual couples trying to conceive across the US and Canada and found that men who used marijuana at least once a week faced roughly twice the chance of their partner's pregnancy ending in miscarriage compared to men who did not use cannabis. This finding supports the theory that cannabis use may affect the earliest stages of pregnancy by impacting sperm quality. However, other research suggests that even once-a-week users may harm their chances of producing healthy offspring. Despite these findings, Dr. Natalie Crawford, a reproductive endocrinologist, called the cannabis use "hugely detrimental" to sperm production and quality.