Jan. 7, 2008 -- When a man fathers his first child, his lifetime risk of prostate cancer goes up 16%.
Nobody has any idea why this might be so, but the finding, based on 51.6 million man-years of data, is as certain as any statistical fact can be.
Here's another piece of the puzzle: After a man fathers a second child, each new child cuts his prostate cancer risk by 5%.
Why these things might happen is a mystery even to the researchers who discovered the disturbing factoids. But facts they are, says Morten Frisch, MD, PhD, DSc, senior investigator at the Statens Serum Institute in Copenhagen, Denmark.
"This is very, very, very statistically significant," Frisch tells WebMD. "It could be the one needle in the haystack; just a statistical anomaly. But we don't believe that. The findings are very comparable to a larger study in Sweden and also to other studies. I do not think you should attribute this to chance."
Why might fathering a child increase prostate cancer risk? Why should three or more children diminish this risk?
"This is very puzzling," Frisch says. "We have no good biological data to give us a hint as to why these two apparently divergent findings come up. Probably there are two different explanations."
Frisch and colleagues published their findings in the Feb. 15 issue of the American Cancer Society's journal Cancer. So WebMD asked ACS chief medical officer Otis Brawley, MD, to explain the findings.
"I still wonder about this finding itself," Brawley says. "Nobody can recommend that men not have kids to prevent having prostate cancer. It may be there is something about guys with good sperm motility having a higher risk of prostate cancer, although I don't believe that to be true."
The true importance of the finding, if any, may not become clear for years. Brawley points to the example of the 1930s finding that nuns rarely got cervical cancer -- and that many of the few nuns who did were reformed "bad girls."
Now, Brawley says, we know that cervical cancer can be caused by human papillomavirus, a sexually transmitted infection not yet linked to cancer in the 1930s.
Frisch notes that the study disproves an earlier finding that men who do not have sons are at increased risk of prostate cancer. The Danish data shows that the sex of a man's offspring does not affect his prostate cancer risk.