Pregnancy incidence was 10.0 per 100 person-years among women assigned placebo , 11.9 among those assigned TDF ( incidence difference , 1.9 ; 95 % CI , -1.1 to 4.9 [ P = .22 vs placebo ] ) , and 8.8 among those assigned FTC+TDF ( incidence difference , -1.3 ; 95 % CI , -4.1 to 1.5 [ P = .39 vs placebo ] ) .