"Exposure to short duration gravity load changes including microgravity, as sustained in a parabolic flight statistically significantly decreases the sperm motility and vitality of human fresh sperm samples," the team found, adding that this may have huge importance for any prolonged human settlement missions in space. 

"In the future, should humans remain in space for long periods of time with exposure to different microgravity and hypergravity peaks, which could range from months to a number of years, reproduction may pose a problem to be tackled."

The mechanism by which sperm motility was decreased remains unknown, with further study needed.