Table 2.

Relation between parity and spontaneous preterm birth < 37, < 32, < 28 and between 34 and 37 weeks of gestation

	Unadjusted OR (95% CI)	A*: Adjusted OR (95% CI)	B**: Adjusted OR (95% CI)
PTB < 37 weeks
P0	1.83 (1.78–1.88)	1.95 (1.89–2.00)	1.93 (1.88–1.98)
P1 (ref)	1.0	1.0	1.0
P2	0.94 (0.90–0.99)	0.92 (0.88–0.97)	0.93 (0.89–0.97)
P3	1.06 (0.99–1.13)	1.00 (0.93–1.08)	1.01 (0.94–1.08)
P4	1.34 (1.20–1.50)	1.26 (1.13–1.41)	1.27 (1.14–1.42)
PTB < 32 weeks
P0	2.04 (1.89–2.21)	2.15 (1.98–2.33)	2.19 (2.02–2.38)
P1 (ref)	1.0	1.0	1.0
P2	1.10 (0.97–1.25)	1.05 (0.92–1.19)	1.06 (0.93–1.20)
P3	1.32 (1.09–1.59)	1.15 (0.95–1.40)	1.17 (0.97–1.42)
P4	2.05 (1.57–2.67)	1.72 (1.31–2.25)	1.76 (1.34–2.31)
PTB < 28 weeks
P0	1.95 (1.72–2.20)	2.02 (1.78–2.29)	2..11 (1.86–2.39)
P1 (ref)	1.0	1.0	1.0
P2	1.07 (0.89–1.30)	1.00 (0.82–1.21)	1.03 (0.85–1.25)
P3	1.65 (1.27–2.16)	1.38 (1.05–1.81)	1.44 (1.10–1.88)
P4	3.10 (2.21–4.35)	2.44 (1.73–3.45)	2.59 (1.84–3.66)
PTB 34–37 weeks
P0	1.75 (1.70–1.80)	1.85 (1.79–1.91)	1.83 (1.77–1.89)
P1 (ref)	1.0	1.0	1.0
P2	0.91 (0.87–0.96)	0.90 (0.85–0.94)	0.90 (0.85–0.95)
P3	1.00 (0.93–1.09)	0.97 (0.89–1.05)	0.97 (0.90–1.06)
P4	1.22 (1.08–1.39)	1.17 (1.03–1.33)	1.18 (1.03–1.34)

Abbreviations: OR odds ratio; CI confidence interval; PTB preterm birth

^*Correction model A: Adjusted for maternal age (< 20 years, ≥40 years and continuous), non-White ethnicity, low socioeconomic status, and a prior preterm birth

^**Correction model B: Adjusted for A and artificial reproductive techniques, male fetal gender, hypertension, preeclampsia, and small for gestational age < p10