In pregnancy, cART is considered optimal for maternal health and for preventing the emergence of resistance that could compromise further care. In Canada, the majority of HIV-positive pregnant women receive a PI-based cART regimen. In the past, therapy was generally deferred until after the first trimester (if not required for maternal health) to minimize any unknown risk of teratogenicity. However, as treatment is now started earlier in HIV infection and as perinatal transmission rates are lowest in those with prolonged suppression of viral load during pregnancy, women are increasingly commencing cART either before conception or earlier in pregnancy.