London

Are today's young men less fertile than their fathers were? It's a controversy in the fertility field, with some experts raising the alarm over what some are calling a "sperm crisis" because they believe men's sperm counts have been decreasing for a decade or more.

Experts here for the European Society of Human Reproduction and Embryology annual conference last week debated the issue for an entire day.

One recent analysis found that in France, the sperm concentration of men decreased by nearly one-third between 1989 and 2005. Most but not all studies from several European nations with large databases and the ability to track health records have found that over the past 15 years or so, the counts of healthy men ages 18 to 25 have significantly decreased. This comes after a prominent study from the 1990s suggested that sperm count has decreased by half over the last half-century.

Many experts questioned the validity of those findings. There are huge variations in results by country and region. Certain areas, especially in the developing world, haven't been studied at all. In the U.S., some historical data suggest a decrease in sperm count among American men, but no published recent data exist.