http://www.msnbc.msn.com/id/8901143/

Updated: 7:32 p.m. ET Aug. 10, 2005
LONDON - One in 25 fathers could unknowingly be raising another man’s child, British scientists said on Thursday.

Bellis said that while mix-ups of semen during artificial insemination accounted for some cases of paternal discrepancy, the majority were due to a woman having sexual relationships outside marriage.