penis size has been linked to infertility in a study. 

Scientists at the University of Utah, Salt Lake City, set out to answer whether penis size affects a man's fertility, and found that men with organs measuring 12.5 centimetres on average struggled to conceive compared with those who measured 13.4 centimetres. This is believed to be the first study to make the association between fertility and penis length. 

According to the U.S. Department of Health, infertility is defined as when a couple cannot conceive after one year of unprotected intercourse. The causes of infertility can be varied and accumulative, but testicular problems are the most common cause in men followed by hormone problems and blockages. In half of the cases, no definitive cause can be pinpointed.