Table 3. Maternal characteristicsa used to weight women according to the type of recurrent prematurity. Brazil, 2011–2012.

	Spontaneous recurrent preterm birth^b (n = 197)	p^c	Recurrent provider-initiated preterm birth^b (n = 136)	p^c

	Crude OR (95%CI)		Crude OR (95%CI)
Maternal age
12 to 19 years	2.08 (1.60–2.71)	< 0.001	0.76 (0.47–1.24)	0.991
20 to 34 years	1.00		1.00
≥ 35 years	1.02 (0.81–1.27)	0.912	1.74 (1.39–2.19)	< 0.001
Socioeconomic classification
Class D/E – low	1.00		1.00
Class C – middle	0.73 (0.61–0.88)	< 0.001	1.29 (1.00–1.68)	0.063
Class A/B – high	0.51 (0.40–0.64)	< 0.001	1.89 (1.43–2.51)	< 0.001
Smoking in the third trimester of pregnancy
No	1.00		-
Yes, < 10 cigarettes/day	1.36 (0.98–1.90)	0.070	-	-
Yes, ≥ 10 cigarettes/day	1.74 (1.22–2.47)	0.002	-	-
Parity
1–2 previous deliveries	1.00		1.00
≥ 3 previous deliveries	1.43 (1.18–1.73)	< 0.001	1.32 (1.04–1.67)	0.023
Previous cesarean section
No	-	-	1.00
Yes	-	-	2.30 (1.89–2.81)	< 0.001
Previous stillbirth or neonatal death
No	1.00		1.00
Yes	2.21 (1.70–2.87)	< 0.001	2.82 (2.11–3.76)	< 0.001
Chronic hypertension
No	1.00		1.00
Yes	1.01 (0.63–1.59)	0.990	3.06 (2.12–4.40)	< 0.001
Chronic diabetes
No	1.00		1.00
Yes	1.31 (0.67–2.57)	0.447	3.09 (1.74–5.49)	< 0.001

OR: odds ratio; 95%CI: 95% confidence interval.

^a All variables were selected based on the DAG (directed acyclic graph).

^b Different due to missing values.

^c The outcomes were compared with the category: 39–40 weeks gestation.