There are 5 major periods of concern when considering use of an antimicrobial agent, or other drug, during pregnancy. The preconception period is important for both males and females because drug administration to either gender in animal models can affect embryonic development or result in embryonic death. Use of drugs during the first 7 days after conception probably represents an all-or-none effect on the fertilized ovum: Either the damage is so great that the embryo dies, or the damaged cells are replaced by undifferentiated cells that develop normally. Once implantation occurs, after 14 days, differentiation of the embryo begins. This is the period of time when most drug-associated congenital abnormalities are produced. Therefore, it is recommended to avoid any unnecessary use of medications during the first trimester. Exposure during the second and third trimesters presents a smaller risk because most organ systems are developed by this time.The last period of concern is during breast-feeding.All antimicrobial agents administered to the lactating mother can be detected in her milk, but detrimental effects have only occasionally been encountered.