Context: The morbidity potentially associated with unverified penicillin allergy in pregnant women, with and without group B streptococcus (GBS) infections, is unknown. Penicillin allergy testing is safe during pregnancy but is done infrequently.

Objective: To determine morbidity associated with antibiotic use in a large cohort of pregnant women, with and without an unverified history of penicillin allergy, and with and without GBS.

Design: Retrospective. All pregnant women who delivered live infants in Kaiser Permanente Southern California between January 1, 2009, and December 31, 2014, were identified.

Main outcome measures: Penicillin allergy status at delivery, delivery method, maternal and infant hospital utilization, peripartum antibiotic exposures, new antibiotic-associated adverse drug reactions, and new Clostridium difficile infections.

Results: There were 170,379 unique women who had 201,316 pregnancies during the study period. There were 16,084 pregnancies in women with an active, but unverified, penicillin allergy at delivery. There were 42,524 pregnancies in GBS-positive women, and 3500 also had a penicillin allergy. Women with a penicillin allergy, with or without GBS, had significantly (about 10%) higher cesarean section rates and spent significantly more (about 0.1) days in the hospital after delivery. Among GBS-positive women, those with an unverified penicillin allergy were exposed to significantly more cefazolin, clindamycin, vancomycin, and gentamicin and had significantly higher rates of adverse drug reactions associated with all antibiotic use.

Conclusions: Unverified penicillin allergy is associated with more hospital utilization and additional morbidity. Penicillin allergy testing of pregnant women with a history of penicillin allergy may help reduce these unwanted outcomes.