The purpose of this study is to evaluate the pharmacokinetics (PKs) of antiretroviral (ARV) and tuberculosis (TB) medications in pregnant women and their infants. (Pharmacokinetics are the various interactions between a drug and the body.) This study will also evaluate the PKs of certain ARVs in postpartum women before and after starting hormonal contraceptives. The PKs of these drugs will be evaluated by measuring the amount of medicine present in blood and/or vaginal secretions.