This relationship was primarily due to a significant effect of selenium among primiparous women ( RR , 2.24 ; 95 % CI , 1.30-3 .86 ; P < 0.01 ) , but not multiparous women ( RR , 1.14 ; 95 % CI , 0.81-1 .59 ; P = 0.54 ) ( P-value for interaction = 0.02 ) .