Table 5. Factors for recurrence of preterm birth delivery by women characteristics in previous pregnancy (N = 5,946).

Variables	cRR,95% CI	aRR,95% CI	P-value
Birth type
Term delivery	1	1
Preterm delivery	2.51[2.11,2.97]	1.85[1.49,2.31]	<0.001
Pre-eclampsia
No	1	1
Yes	1.77[1.29,2.41]	1.46[1.07,2.00]	0.017
Inter-pregnancy interval
Normal IPI	1	1
Short IPI	1.19[1.01,1.40]	1.13[0.96,1.33]	0.148
Long IPI	1.24[1.02,1.51]	1.22[1.01,1.49]	0.042
Clinical subtypes
Spontaneous delivery	1	1
Medically indicated	1.40[1.03,1.88]	1.37[1.00,1.86]	0.048
Term delivery	0.45[0.36,0.57]	0.59[0.46,0.77]	<0.001

cRR: Crude risk ratio, aRR: Adjusted risk ratio