“…Among these, 6,906 deliveries for which the prefecture of the mother’s residence was different from the location of the birth hospital were excluded to avoid exposure misclassification due to the Japanese satogaeri custom (some mothers go back to their parental homes, and deliver at a hospital near their parents’ homes). In addition, the following deliveries were excluded: 2,832 deliveries with gestational age <32 weeks (unsuitable for FHR assessment 23 , 24 ); 9,752 deliveries with missing information on umbilical cord blood pH (n = 7,282) or umbilical blood pH < 7.2 (acidemia) 20 , 25 (n = 2,470); 1,385 deliveries with Apgar scores <7 at 1 minute or 5 minutes after birth (to restrict the study sample to newborns in good health 21 ); 1,726 deliveries to mothers with intrauterine infection, oligohydramnios, or placental abruption (which are causes of FHR monitoring abnormalities 26 , 27 ; one delivery with fetal death (stillbirth); 1,346 deliveries with conception date ≥22 weeks before the study period began and those with conception dates ≤42 weeks before the study period ended to prevent fixed cohort bias 28 , and 105 deliveries with missing data on maternal age (n = 2) or exposure to pollutants (n = 103). After applying these exclusions, 23,782 singleton deliveries at gestational age ≥32 weeks were included in the analysis.…”