Research from two universities in England suggests using cannabis could hurt a man's chances of having children.

A paper published in the June 5 issue of the journal Human Reproduction and based on the largest-ever study to investigate sperm morphology, how lifestyle factors affect the size and shape of sperm, notes young men who use cannabis may be putting their fertility at risk by inadvertently affecting the size and shape of their sperm.

Research teams from the University of Sheffield and also University of Manchester found young men -- aged 30 and younger -- who reported using cannabis within three months prior to testing failed to produce sperm samples considered normal by current medical definitions.

Put another way, "our data suggests that cannabis users might be advised to stop using the drug if they are planning to try and start a family," lead study author Allan Pacey, a senior lecturer in andrology at Sheffield, said in a news release.

The study recruited 2,249 men from 14 fertility clinics throughout the United Kingdom and asked them to fill out detailed questionnaires about their medical histories and lifestyles.

Pacey said he and his colleagues were only able to obtain reliable data about sperm morphology from 1,970 of the participants.

Therefore, the study was divided into two groups: 318 men -- who produced sperm of which less than 4 percent was the correct size and shape -- and a control group of 1,652 men with "normal" sperm samples, where more than 4 percent of the individual cells met the criteria for correct size and shape.

Previous research has suggested only individual sperm cells with good morphology are able to pass into a woman's body following sex and swim their way to an egg for fertilization.

Laboratory studies further indicated sperm cells with poor morphology swim less well because their abnormal shapes makes them less efficient.

The latest survey showed men who produced sperm with less than 4 percent normal cells sperm were nearly twice as likely to have produced the samples in question during summer months -- June, July and August -- or, if they were younger than 30 years old, if they had used cannabis in the three month period before providing their samples.

The research discovered sperm size and shape were generally worse in samples taken in the summer months, although better in men who had abstained from sexual activity for more than six days before the sampling.

Other common lifestyle factors reported by men, including smoking cigarettes and drinking alcohol, appeared to have little effect on sperm quality.

"Our knowledge of factors that influence sperm size and shape is very limited, yet faced with a diagnosis of poor sperm morphology, many men are concerned to try and identify any factors in their lifestyle that could be causing this," said Pacey. "It is therefore reassuring to find that there are very few identifiable risks."

Even though, beyond external climate and use of cannabis, the study failed to find any link between sperm morphology and other common lifestyle factors, the researchers believe behaviors such as smoking and drinking alcohol could correlate with other aspects of sperm that were not measured, such as the quality of the DNA contained in heads of individual sperm cells.