Fetal heart rate (FHR) monitoring is essential for fetal management during pregnancy and delivery but results in many false-positive diagnoses. Air pollution affects the uterine environment; thus, air pollution may change FHR reactivity. This study assessed the association between exposure to air pollution during pregnancy and FHR monitoring abnormalities using 2005-2010 data from the Japan Perinatal Registry Network database. Participants were 23,782 singleton pregnant women with FHR monitoring, without acidemia or fetal asphyxia. We assessed exposure to air pollutants, including particulate matter (PM), ozone, nitrogen dioxide (NO2), and sulfur dioxide (SO2). In a multi-trimester model, first-trimester PM exposure was associated with false positives in FHR monitoring (odds ratio [OR] per interquartile range (10.7 μg/m3) increase = 1.20; 95% CI: 1.05-1.37), but not second-trimester exposure (OR = 1.05; 95% CI: 0.91-1.21) and third-trimester exposure (OR = 1.06; 95% CI: 0.96-1.17). The association with first-trimester PM exposure persisted after adjustment for exposure to ozone, NO2, and SO2; however, ozone, NO2, and SO2 exposure was not associated with false positives in FHR monitoring. First-trimester PM exposure may alter fetal cardiac response and lead to false positives in FHR monitoring.
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