Table 4.

Logistic regression analyses for the interaction effects between chlamydia, gonorrhea, and syphilis (Factor 1) and prenatal care (PNC) utilization status (Factor 2) on preterm birth.

Factor 1	Factor 2	No Preterm Birth	Preterm Birth		p Value
		n	n	aOR (95%CI)
Chlamydia (−)	PNC (−)	45,127	12,999	3.11 (3.05, 3.18)	<0.001
Chlamydia (−)	PNC (+)	3,034,729	260,730	1.00
Chlamydia (+)	PNC (−)	1004	384	3.56 (3.15, 4.02)	<0.001
Chlamydia (+)	PNC (+)	56,962	6093	1.02 (0.99, 1.05)	0.229
Multiplicative Interaction				1.12 (1.01, 1.25)	0.054
Relative Excess Risk due to Interaction (RERI)				0.43 (0.32, 0.53)	<0.001
Attributable Proportion (AP)				0.12 (0.02, 0.22)	0.023
Synergy Index (SI)				1.20 (1.10, 1.31)	<0.001
Gonorrhea (−)	PNC (−)	45,878	13,232	3.11 (3.05, 3.18)	<0.001
Gonorrhea (−)	PNC (+)	3,082,379	265,626	1.00
Gonorrhea (+)	PNC (−)	253	151	5.06 (4.10, 6.25)	<0.001
Gonorrhea (+)	PNC (+)	9312	1197	1.12 (1.05, 1.19)	0.000
Multiplicative Interaction				1.45 (1.22, 1.72)	0.032
Relative Excess Risk due to Interaction (RERI)				1.83 (1.66, 2.00)	<0.001
Attributable Proportion (AP)				0.36 (0.19, 0.53)	<0.001
Synergy Index (SI)				1.82 (1.65, 1.99)	<0.001
Syphilis (−)	PNC (−)	45,976	13,289	3.12 (3.05, 3.18)	<0.001
Syphilis (−)	PNC (+)	3,087,591	266,188	1.00
Syphilis (+)	PNC (−)	155	94	5.54 (4.24, 7.24)	<0.001
Syphilis (+)	PNC (+)	4100	635	1.30 (1.20, 1.42)	<0.001
Multiplicative Interaction				1.36 (1.10, 1.70)	0.040
Relative Excess Risk due to Interaction (RERI)				2.12 (1.90, 2.34)	<0.001
Attributable Proportion (AP)				0.38 (0.16, 0.60)	0.000
Synergy Index (SI)				1.88 (1.66, 2.09)	<0.001

n: number; aOR: adjusted odds ratio; CI: confidence interval; PNC (+): prenatal care adequate; PNC (−): prenatal care inadequate; Adjusted for: Race/ethnicity, age, education, marital status, health insurance, parity, prior preterm birth, gestational diabetes, gestational hypertension, smoking, and infant sex; RERI: Relative Excess Risk due to Interaction; AP: Attributable Proportion; SI: Synergy Index.