The U.S. Food and Drug Administration (FDA) has classified most antibiotics as belonging in either category B or category C. Category B indicates that either animal studies have shown no risk to an animal fetus, but there have been no human studies, or that animal studies have revealed a risk to an animal fetus but human studies have revealed no risk. Category C indicates that animal studies have shown a risk to animal fetuses, but the drugs have not been adequately studied in humans.

The use of antibiotics during pregnancy or while breastfeeding depends on the nature of the infection and the potential benefits and risks to both the mother and baby. In many cases, the benefit of treating an infection may outweigh a slight or theoretical risk associated with the medication.

Certain classes of antibiotics generally have been considered safe throughout pregnancy, including the penicillins and cephalosporins. Others should be avoided during certain trimesters because they may interfere with embryonic or fetal development. Tetracyclines are usually avoided during the first trimester because they may affect developing bones. In the second trimester, aminoglycosides may be toxic to auditory development in the fetus. Still other antibiotics should be avoided throughout pregnancy, such as quinolones.

During the third trimester, a pregnant woman may be tested for group B strep infection. This infection may cause no problems but may be passed to the infant during delivery. Women who test positive for the infection are given antibiotics during labor to prevent transmission to the baby.

Similar benefits and risks apply to antibiotic use while breastfeeding. Some antibiotics pass into breast milk and others do not. During pregnancy, most of the medications are processed by the mother's liver and kidneys. After birth, the infant's liver and kidneys must filter the medications that pass through breast milk. The filtering capacity of the infant's organs are much lower than those of the mother. A woman may be advised not to breastfeed while taking certain antibiotics.

Pregnant and breastfeeding women should consult their physicians about any antibiotic use.