Table 2 Crude and adjusted odds ratios (with 95% confidence intervals in parentheses) for the prevalence of preterm birth according to birth year and maternal characteristics in California for the period 2007–2016

0.99 (0.98–1.00)

0.99 (0.97–1.00)

0.95 (0.93–0.96)

0.94 (0.92–0.96)

0.93 (0.91–0.94)

0.92 (0.91–0.94)

0.92 (0.90–0.93)

0.91 (0.90–0.93)

0.92 (0.90–0.93)

0.91 (0.90–0.93)

0.91 (0.89–0.92)

0.90 (0.89–0.92)

0.89 (0.87–0.90)

0.89 (0.87–0.90)

0.92 (0.90–0.93)

0.91 (0.89–0.93)

0.94 (0.92–0.95)

0.93 (0.91–0.94)

Ref^b

Ref^b

1.16 (1.15–1.18)

1.07 (1.05–1.09)

0.97 (0.96–0.98)

1.08 (1.07–1.10)

1.01 (1.00–1.02)

1.24 (1.22–1.26)

1.20 (1.19–1.22)

1.51 (1.49–1.54)

1.54 (1.52–1.57)

1.92 (1.88–1.96)

Ref^b

Ref^b

1.77 (1.74–1.79)

1.64 (1.61–1.67)

1.46 (1.39–1.54)

1.26 (1.18–1.35)

1.16 (1.14–1.17)

1.40 (1.38–1.42)

1.23 (1.22–1.24)

1.22 (1.21–1.24)

1.26 (1.23–1.29)

1.23 (1.20–1.26)

1.35 (1.32–1.38)

1.26 (1.22–1.30)

1.48 (1.41–1.55)

1.43 (1.35–1.51)

Ref^b

Ref^b

1.37 (1.36–1.38)

1.35 (1.33–1.38)

1.28 (1.27–1.29)

1.30 (1.28–1.31)

1.27 (1.26–1.29)

1.28 (1.26–1.30)

1.39 (1.36–1.41)

1.30 (1.27–1.34)

Ref^b

Ref^b

1.08 (1.07–1.08)

1.15 (1.14–1.16)

Ref^b

Ref^b

0.97 (0.95–1.00)

1.01 (0.98–1.04)

1.03 (1.00–1.05)

1.04 (1.01–1.06)

1.14 (1.11–1.16)

1.13 (1.10–1.16)

1.13 (1.11–1.15)

1.12 (1.09–1.14)

0.99 (0.97–1.02)

1.05 (1.02–1.08)

1.03 (1.01–1.06)

1.05 (1.02–1.08)

1.07 (1.04–1.09)

1.13 (1.10–1.16)

1.21 (1.18–1.24)

1.18 (1.15–1.22)

Ref^b

Ref^b

1.13 (1.12–1.14)

1.12 (1.11–1.13)

Ref^b

Ref^b

0.95 (0.94–0.96)

1.13 (1.12–1.14)

Ref^b

Ref^b

1.08 (1.07–1.09)

1.07 (1.06–1.09)

Ref^b

Ref^b

1.03 (1.02–1.04)

1.16 (1.15–1.17)

1.84 (1.80–1.88)

1.35 (1.32–1.39)

Ref^b

Ref^b

1.85 (1.81–1.89)

1.57 (1.53–1.62)

Ref^b

Ref^b

1.23 (1.20–1.25)

1.23 (1.20–1.25)

1.09 (1.08–1.09)

1.06 (1.05–1.07)

1.22 (1.21–1.23)

1.16 (1.15–1.17)

1.31 (1.29–1.33)

1.24 (1.22–1.26)

1.44 (1.42–1.47)

1.33 (1.30–1.36)

Ref^b

Ref^b