In the world's largest study to investigate how common lifestyle factors influence the size and shape of sperm (referred to as sperm morphology), a research team from the Universities of Sheffield and Manchester also found that sperm size and shape was worse in samples ejaculated in the summer months but was better in men who had abstained from sexual activity for more than six days.
For the study, scientists recruited 2,249 men from 14 fertility clinics around the UK1 and asked them to fill out detailed questionnaires about their medical history and their lifestyle.
Reliable data about sperm morphology was only available for 1,970 men and so the researchers compared the information collected for 318 men who produced sperm of which less than four per cent was the correct size and shape and a control group of 1,652 men where this was above four per cent and therefore considered 'normal' by current medical definitions.
Men who produced ejaculates with less than four percent normal sperm were nearly twice as likely to have produced a sample in the summer months (June to August), or if they were younger than 30 years old, to have used cannabis in the three month period prior to ejaculation.
The study has been published in the medical journal Human Reproduction.