Complex sentence: One measure of infidelity among couples is the frequency of children secretly conceived with a different partner, leading to "non-paternities"; such covertly illegitimate children amount to about 1-2% of all newborns in Western populations.
Split into simpler sentences: One measure of infidelity among couples is the frequency of children secretly conceived with a different partner, leading to "non-paternities". Such covertly illegitimate children amount to about 1-2% of all newborns in Western populations.