Working in a physically demanding job, having high blood pressure, and taking multiple medications are among the health risks that may undermine a man’s fertility, according to a new study.

"Nearly 15 percent of US couples do not become pregnant in their first year of trying," said the study’s senior author Germaine Buck Louis of the National Institutes of Health (NIH). 

Semen quality is a measure of a man’s ability to achieve fertilisation and is based on the number, shape and movement ability of sperm as well as other factors.

"As men are having children later in life, the importance of diseases we once thought as separate from fertility must be re-explored," said principal investigator Michael L. Eisenberg of Stanford University, California. 

The study group comprised 456 men with an average age of 31.8 years. More than half had never fathered a pregnancy. 

The researchers found that 13 percent of the men who reported heavy work-related activity had lower sperm counts, compared to 6 percent of the men who reported no workplace exertion. 

The researchers found that those with high blood pressure had a lower percentage of normally shaped sperm, compared to men who reported no high blood pressure.

The researchers observed that the more medications a man reported taking, the higher his risk of a low sperm count. 

"The good news is that these factors, if they are confirmed to have negative effects on male fertility, can potentially be modified by medical care or changing job-related behaviours," said NIH’s Dr. Buck Louis. 

The results were published online in Fertility and Sterility.