Are young men today less fertile than their fathers and grandfathers were?

Sperm counts in men from North America, Europe, Australia and New Zealand have dropped by more than 50 percent in a nearly 40-year period, according to a new analysis.

The international team of researchers said the rate of decline showed no sign of leveling off and might be considered as a "canary in the coal mine" for male health across the lifespan.

Researchers led by Dr. Hagai Levine of Hebrew University of Jerusalem examined 185 sperm-count studies from 1973 and 2011, including samples from 42,935 men. The results, published in the journal Human Reproduction Update, showed a 52 percent decline in sperm concentration and a 59 percent decline in total sperm count.

The analysis didn't explore reasons for the decline, but researchers point out that dropping sperm counts have been linked to environmental and lifestyle factors, including smoking, diet and exposure to certain chemicals and pesticides.

Fertility was also the hot question under debate at the European Society of Human Reproduction and Embryology annual conference several years ago. Some health experts argued that men today are facing a "sperm crisis" with sperm counts that have been decreasing for at least the last decade, reported The Wall Street Journal.

A study in France found that the sperm concentration of men decreased by nearly one-third between 1989 and 2005. This follows the findings of several other European health studies that have found that over the past 15 years or so, the sperm counts of healthy men ages 18 to 25 have significantly decreased.

If there is a sperm crisis, as many health experts believe, it goes beyond implications of male fertility. Sperm counts could be an overall indicator of a man's health, and a low sperm count could be caused by or linked to a whole host of health implications.

Editor's note: This story has been updated since it was originally published in July 2013.