Background Understanding the absolute and relative risk of human immunodeficiency virus type 1 (HIV) acquisition during pregnancy and the postpartum period can inform HIV prevention strategies for women. Methods We used a complementary log-log model and data from 2751 HIV-serodiscordant couples to compare the probability of HIV acquisition among women per sex act during early pregnancy, late pregnancy, the postpartum period, and the nonpregnant period. Results At total of 686 pregnancies were identified, and 82 incident HIV infections occurred. After adjustment for condom use, age, preexposure prophylaxis (PrEP) use, and HIV viral load, the per-act probability of HIV acquisition was higher in late pregnancy (adjusted relative risk [aRR], 2.82; P =.01) and the postpartum period (aRR, 3.97; P =.01) as compared to that during nonpregnant period. For a 25-year-old woman not taking PrEP, the HIV acquisition probability per condomless sex act with an HIV-infected male partner with a viral load of 10 000 copies/mL was 0.0011 (95% confidence interval [CI] 0.005-0.0019), 0.0022 (95% CI, 0.0004-0.0093), 0.0030 (95% CI, 0.0007-0.0108), and 0.0042 (95% CI, 0.0007-0.0177) during the nonpregnant period, early pregnancy, late pregnancy, and the postpartum period, respectively. Conclusion The HIV acquisition probability per condomless sex act steadily increased during pregnancy and was highest during the postpartum period, suggesting that biological changes during pregnancy and the postpartum period increase HIV susceptibility among women.
