To investigate the association between air travel during early pregnancy and the risk of spontaneous miscarriage.

This population-based retrospective cohort study included completed singleton pregnancies ending in either a first-trimester spontaneous miscarriage or a live birth between 2010 and 2019. Only pregnancies with a confirmed last menstrual period (LMP) date were included to ensure accurate temporal assessment of air travel in relation to pregnancy outcome.

Singleton pregnancies resulting in live birth were compared with those ending in spontaneous miscarriage during the first trimester. Air travel between 4 + 0 and 13+6 weeks of gestation was defined as the primary exposure. Spontaneous miscarriage was identified using ICD-9 codes. Multivariable logistic regression was performed to estimate adjusted odds ratios (aOR) with 95% confidence intervals (CI).

A total of 439,990 pregnancies were analyzed, including 427,727 live births (97.2%) and 12,263 miscarriages (2.8%). Air travel during the first trimester was more common among women who experienced miscarriage compared to those with live births (0.01% vs. 0.00,014 %, P < 0.001). After adjustment for potential confounders, air travel was associated with more than fivefold higher odds of miscarriage (aOR 5.20, 95% CI 4.38–6.33).

Air travel during the first trimester was associated with an increased risk of spontaneous miscarriage. While causality cannot be established, these findings may have implications for counseling women considering air travel during early pregnancy.