Some antibiotics may link to miscarriage risk
Common urinary infection antibiotics were not linked to higher miscarriage risk, while several other antimicrobial drugs showed higher risk that may partly reflect the infection being treated.
*Population-based cohort study; Level 2b (OCEBM).

Citation

Boissiere-O'Neill T, van Gelder MMHJ, Engjom HM, Nordeng HME. Exposure to systemic antimicrobials during pregnancy and risk of miscarriage: a population-based registry study. BJOG: An International Journal of Obstetrics & Gynaecology. 2026;133:1394–1404. doi:10.1111/1471-0528.181551.

Background

Antimicrobial medicines are frequently used in early pregnancy, but past studies have been hard to interpret because infection itself may raise miscarriage risk and because of timing-related biases.

Patients

704,082 pregnancies in Norway (2009–2018). Excluded ectopic or molar pregnancies and pregnancies exposed to known birth-defect–causing medicines near conception.

Intervention

Filled prescriptions for whole-body (not topical) antibacterial, antifungal, or antiprotozoal medicines in early pregnancy, treated as changing over time; prescriptions within 14 days before a pregnancy outcome were not counted.

Control

No filled prescription for these medicines during the same time window.

Outcome

Miscarriage before 20 weeks of pregnancy, identified from linked national registries.

Follow-up Period

From last menstrual period to 20 weeks.

Results

Exposure (weighted analysis) Association with miscarriage Hazard ratio (95% confidence interval)
Nitrofurantoin Lower risk 0.75 (0.66–0.84)
Pivmecillinam Lower risk 0.91 (0.87–0.95)
Metronidazole Higher risk 2.00 (1.82–2.21)
Ciprofloxacin Higher risk 1.89 (1.62–2.20)
Cephalexin Higher risk 1.87 (1.57–2.22)
Fluconazole (oral) Higher risk 1.61 (1.45–1.78)
Trimethoprim with sulfa medicine Higher risk 1.49 (1.36–1.63)
Elective terminations were treated as censoring events. Bias analyses suggested several drug–miscarriage links could be explained by the underlying infection.

Limitations

Observational data cannot fully separate medicine effects from infection severity. Very early miscarriages not seen in healthcare records were missed. Some antifungal use was over-the-counter and not captured. A filled prescription may not mean the medicine was taken.

Funding

University of Oslo; Norwegian Research Council visiting grant.

Clinical Application

When treating early-pregnancy infections, prefer nitrofurantoin or pivmecillinam for urinary infection when appropriate; be cautious with oral fluconazole, metronidazole, quinolones, and cephalexin.