It is generally considered safe to take erythromycin during pregnancy because there is minimal risk of harmful effects to the developing fetus. Erythromycin is placed in Pregnancy Category B by the Food and Drug Administration (FDA), which means that animal testing of erythromycin during pregnancy has found no significant risk to the fetus. While no full-controlled human studies have been conducted, no significant teratogenic, or harmfully abnormal, effects have been found in humans. Taking a specific form of erythromycin called erythromycin estolate during pregnancy, however, has been found to increase the possibility of hepatotoxicity, or liver damage to the mother, and it should only been used when a clear need has been demonstrated.
During an animal study, female rats were given erythromycin during pregnancy until after weaning had been completed. In this study, no harmful or teratogenic effects were observed, even when the rats were given erythromycin for the entire mating process as well as during pregnancy and weaning. This evidence supports the safety of erythromycin, but it cannot be fully applied to humans.
Several surveillance studies have observed various pregnancies and recorded data about the safety of using erythromycin during pregnancy. One study, the Collaborative Perinatal Project, observed 230 pregnancies in which the developing fetus had been exposed to erythromycin, out of a total of 50,282 pregnancies. No major link between erythromycin during pregnancy and any major developmental issues was observed.
Another surveillance study, which monitored 229,101 pregnancies of patients receiving Medicaid in the state of Michigan, found 6,972 cases of erythromycin use during pregnancy. From these pregnancies, 320 major birth defects were observed. While this number is slightly higher than the expected average of 297 defects, the study found no significant correlation between erythromycin use and resulting birth defects.
Erythromycin does cross the placental barrier and enters the fetal bloodstream in small amounts. Although the levels of erythromycin increase in the fetal bloodstream when the mother’s dosage increases, maternal erythromycin levels can vary during pregnancy, making it difficult to measure the degree of transmission. The levels in the fetal bloodstream are generally considered sufficiently low enough to not cause harmful effects.
A salt form of erythromycin, erythromycin estolate, should not be used during pregnancy. It has been found to increase the levels of serum glutamic-oxaloacetic transaminase (SGOT) in the bloodstream when taken during the second trimester of pregnancy. In one study, 10% of patients experienced these increased SGOT levels, indicating hepatotoxy had occurred. Once the use of erythromycin was stopped, these levels returned to normal, indicating that this is a temporary side effect of the drug.