We’ve long known that if women take certain HIV meds during pregnancy and childbirth and administer them to their newborns, the risk that the infant will contract HIV falls to near 2 percent. Now there’s evidence that starting meds before pregnancy is an even surer thing: In a study conducted in Johannesburg, women who took HIV meds before they became pregnant had the lowest chance of passing the virus to their newborns.

Among women who didn’t begin taking HIV meds until sometime during pregnancy, the rate of transmission was 5.7 percent. With each additional week the meds were taken before delivery, the transmission rate dropped even further.

In the United States, where rates of HIV transmission to newborns are far lower than in Africa, the study still carries a message. Deborah Cohan, MD, medical director of San Francisco’s Bay Area Perinatal AIDS Center, says, “The key message of this study is that you don’t want to wait too long once you are pregnant to start HIV meds.” Cohan adds that you have to balance benefits with risks—any possible impact of drugs on a developing fetus, say, or the problem of how to tolerate a new drug regimen while experiencing morning sickness. If women haven’t begun HIV treatment pre-pregnancy, Cohan says, “I still recommend starting at around 12 weeks or so, if their nausea is under control.” And for expectant moms with very low CD4 counts, Cohan recommends starting sooner.