Roller-coaster sperm counts - and births.

Danish scientists astounded the fertility community 5 years ago when they reported signs that sperm counts appeared to have fallen globally over the past two generations (SN: 1/22/94, p. 56). The controversy they evoked only became more heated as a host of follow-up studies, conducted elsewhere, alternately confirmed and refuted the suggested trend.

Fertility specialist Harry Fisch of Columbia University's College of Physicians and Surgeons now thinks he's uncovered what's behind many of the apparent contradictions.

Last year, this urologist and his colleagues showed that even within the United States, sperm counts can vary dramatically between regions (SN: 6/8/96, p. 365). In the March Journal of Urology, Fisch's team now reports that in data collected over 24 years in Minnesota, sperm counts fluctuate from a high of 123 million per milliliter (ml) of semen in 1980 to a low of 46.5 million per ml in 1974.

Ironically, Fisch says, whether changes in the average sperm count within a population actually affect regional fertility, as measured by birth rates, had never been formally addressed. In the new study, his team looks at that question and finds that annual Minnesota birth rates indeed tracked trends in sperm counts, with a low of 13.8 live births per 1,000 residents in 1973 and a high of 16.7 in 1981.

Whether the reported data depict a rise or fall, he says, "depends on when you start the study." For instance, he notes, his group last year found U.S. sperm counts to be rising-but only because that analysis began in 1971, when sperm counts were low. Had his team instead initiated its analysis with data for "1977, 1980, or 1984, when mean sperm counts were high," Fisch observes, "we would have likely reported a decrease in sperm counts with time," as have recent studies from Scotland and France (SN: 2/25/95, p. 127).

The Minnesota data, based on sperm that had been banked before the 660 donors underwent vasectomies, constitute the world's longest record for this fertility factor at a given site.

Though no one knows what's behind the sperm count fluctuations, Fisch strongly suspects it may be some environmental factor, such as climate.