Swan's work has shown that certain chemicals in the environment -- such as those associated with plastics and pesticides -- seem to be linked to a lower sperm count in men who have been exposed to those chemicals.
"I think we are looking at a problem that has just recently been identified," Swan said.
The findings aren't just on paper. Swan said that in her study of men living in rural areas of mid-Missouri, the data suggested that pesticides may be responsible for cutting the usable sperms levels of these men in half when compared with their urban counterparts in Minneapolis or New York City.
While some of these factors can be avoided (smoking and heavy alcohol use, for example, are known for their sperm-slaying potential) other fertility hazards may attack when males are most vulnerable -- before they have been born.
According to John McCarrey, professor of cellular and molecular biology at the University of Texas at San Antonio, a host of environmental influences can affect a man's future sperm count even while he is still in the womb.
During fetal development, McCarrey said, "There is a massive reprogramming of many genome functions. There is a lot that's happening during development."
The heart of the matter lies with what he calls the epigenome -- the way in which the body's cells interpret the genetic blueprints laid out by the DNA. Tinker with this at the wrong time, and the results could last a lifetime.
"We're really at the point where we're figuring out what kinds of things have adverse effects," McCarrey said. "At the very least, all of these recent findings have made us much more aware that this is a very sensitive period."
But sensitive to what? Most pregnant women don't go about eating bowls of pesticide, after all.
There could be a number of dietary influences at play. Red meat, for example.
Earlier this year, Swan and her colleagues published a study that suggested the beef that a woman eats while pregnant just might affect the future fertility of her male babies.
"The hypothesis is that hormones put in beef have an effect on sperm count," she said. "If a pregnant woman ate beef more than once a day, her son's sperm count was about 25 percent lower."
And though the effects from a lot of the chemicals in the environment today that men are exposed to are not considered to be persistent, exposures in the womb could have much more lasting effects.
"If the exposure is through the mother when she is pregnant with a male baby, then those changes are permanent," Swan said.
While much of the research on the possible decline of male fertility is still unclear, Swan said there are certain steps men can that may help safeguard their sperm.
"The first caveat is that all of these things have to be tested further, so at no point can we make firm health recommendations," Swan said. "However, if people want to change their behaviors, there are several things that they can do that will probably help and won't hurt."
One of the tips Swan listed is eating organic foods whenever possible. Organic foods, she said, mean less pesticide -- and, in turn, may lead to more viable sperm.
Her second tip: Pregnant women who fear for their future fetus' fertility should avoid heavily using plastic products known to contain phthalates -- softening agents that are suspected to have an effect on sperm count.