Sperm shape and size - known as morphology - were worse in semen samples collected during summer months and from men who smoked marijuana. Morphology was better among samples collected after a man abstained from sexual activity for a few days.
But alcohol drinking and cigarette smoking were not associated with misshapen sperm, researchers found. Neither was a man's weight.
Its always my recommendation to my patients to improve their health as much as possible, Dr. James Smith said.
A study like this doesnt discourage me from saying those things to my patient.
Smith, who was not involved with the new study, is a male reproductive health specialist at the University of California, San Francisco.
Its believed that sperm with the most normal size and shape are most likely to survive in the female reproductive tract, Allan Pacey from the University of Sheffield in the UK and his colleagues write in Human Reproduction. Little is known, however, about the impact that certain lifestyle factors have on sperm morphology.
To learn more, the researchers surveyed men who were seen at one of 14 fertility clinics in the UK about their health and lifestyle habits. They had information on 318 men with abnormal sperm morphology and 1,652 men with normal sperm.
Men were considered to have abnormal sperm morphology if less than four percent of their sperm in a sample of 200 was of normal form, based on guidelines established by the World Health Organization. Any count above four percent is considered normal.
The researchers found that men were about twice as likely to have abnormal sperm if the sample was collected during the summer. They were also more likely to have abnormal sperm if they were young and smoked marijuana.
Although most other medical and lifestyle factors, such as cigarette smoking and alcohol consumption, didnt seem to be linked to sperm morphology, Smith said he still would advise his patients to be as healthy as possible.
Marijuana is certainly a potential worrisome risk factor for hurting sperm quality, he said. Id tell my patients to stop smoking marijuana. I wouldnt say to my patient to go out and do whatever you want because it wont make a difference. To me, that would be overstating those results.
The researchers also caution that the men included in the study may not be representative of all couples with fertility problems.
Smith said a better study would be to focus on whether the couples went on to conceive a child.
What would be great to see is if lifestyle factors - one way or another - are going to be related to getting pregnant, he said.