Sept. 8, 2025

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Urinary tract infections (UTIs) occur when bacteria are in a person’s urine. A UTI that does not cause obvious symptoms is called asymptomatic bacteriuria (ASB). ASB during pregnancy may lead to poor outcomes, such as kidney infection and sepsis in the mother, premature birth, and low birthweight. Due to these risks, doctors screen for ASB during the first trimester prenatal visit. If ASB is present, the UTI is treated with antibiotics.

Some commonly used antibiotics, such as trimethoprim-sulfamethoxazole (TMP-SMX, also known as co-trimoxazole) and nitrofurantoin, have been shown by some studies to potentially act like teratogens. Teratogens are substances that can interfere with fetal development and cause birth defects, similar to how alcohol and smoking are known to harm pregnancies. Many of these studies examining antibiotics relied heavily on patient self-reports and were vulnerable to outside influences, which can skew the results. Further research is needed to determine whether certain antibiotics pose an increased risk of causing birth defects.

A recent study supported by NIH’s Clinical and Translational Science Awards (CTSA) Program examined the potential risks of certain antibiotics during early pregnancy. Researchers at Washington University analyzed data from 71,604 pregnancies between 2006 and 2022. Using a medical claims database, they identified women who were treated early in pregnancy for urinary tract infections with TMP-SMX, nitrofurantoin, fluoroquinolones, or β-lactams, and linked those records to infant health outcomes. The findings showed that TMP-SMX was linked to a higher risk of any birth defect compared with β-lactams.

Michael Kurilla, M.D., Ph.D., director of NCATS’ Division of Clinical Innovation, which oversees the CTSA Program, stated, “Even drugs generally regarded as safe may possess uncommon or rare side effects. The CTSA Program’s focus on analyzing real-world data uncovers any hidden risks and enhances maternal and infant health.”

Currently, the American College of Obstetricians and Gynecologists recommends limiting the use of TMP-SMX and nitrofurantoin during early pregnancy unless no appropriate substitutes are available. The findings from this study — the largest analysis to date — increase our knowledge of how antibiotic use may lead to birth defects and provide more evidence for clinical guidelines.

The results appeared in JAMA Network Open.