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BOSTON -- The risk of HIV transmission per sex act increased steadily through pregnancy, and was highest during postpartum, researchers reported here.

In two HIV prevent studies that included 2,571 African women living in HIV serodiscordant relationships (female partners did not have HIV), the risk of HIV infection was 2.97-fold higher during the second two trimesters of pregnancy (P=0.01) and 4.18-fold higher during the 6-month post partum period (P=0.01), according to Renee Heffron, PhD, of the University of Washington in Seattle, and colleagues.

Among women in serodiscordant relationships in general, the risk of HIV infection is about 5% of the time in 1,000 sexual acts when women were not pregnant, Heffron said at a press conference at the Conference on Retroviruses and Opportunistic Infections.

That increases 2.19-fold for women who are in their first trimester of pregnancy, and then higher for later stage and post partum. "This probably occurs due to biological changes that occur during pregnancy," she said.

Pregnancy incidence was 12.50 per 100 woman-years and 82 HIV transmission events occurred in the studies, Heffron reported.

For the studies, the reference case for HIV transmission probability was a condomless sex act between a woman, age 25, not using pre-exposure prophylaxis (PrEP), and a male partner with HIV RNA of 10,000 copies/ml. If during the study the HIV positive man opted to go on highly active antiretroviral therapy (HAART), then that couple was removed from the trial.

Participants were followed for 2 years with sexual activity and condom use reported monthly. HIV testing and pregnancy testing occurred either quarterly or monthly.

After adjusting for condom use, age, use of PrEP, and HIV viral load, the probability of HIV transmission per sex act was significantly higher in late pregnancy (aRR 2.82, P=0.01) and postpartum (aRR 3.97, P=0.01) versus non-pregnant time, Herron's group reported.

"The risk of HIV transmission per sex act steadily increased through pregnancy and was highest during postpartum, even after accounting for sexual behavior, pre-exposure prophylaxis, and HIV viral load, suggesting that biological changes during these periods increase HIV risk," the authors stated. "While further research is needed to better understand biological susceptibility, scale-up of HIV prevention and testing in antenatal and postpartum care in high HIV prevalence settings is warranted to prevent sexual transmission and identify acute maternal HIV infections."

"During the latter phases of pregnancy and post partum periods are high risk times for HIV infection," concurred Sharon Lewin, MD, of the University of Melbourne in Australia. "I would recommend that my patients use PrEP during those periods."

Lewin, who was not involved in the study, explained that biologic chances in vaginal thickness that occur during pregnancy and post-partum could have a role to play in the increase in infection. "It is well-known that pregnancy is an immuno-depressive state, there are numerous immune-related conditions that improve during pregnancy and flare up after pregnant."

She said that in most serodiscordant couples, the infected partner would be treated with ART, which would markedly reduce the risk of transmission.

"This study is important because it strengthens the importance of diagnosing partners ... the use of PrEP is also advantageous, especially if people are having sex with those other than their partners," Lewin noted. "We know that if the partner was being treated and the woman was taking PrEP, the risk of her becoming infected would be reduced to zero."

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