Factors associated with premature birth: a case-control study

Case-control study, with case group composed of preterm infants and the control group by full term live births. Each case was paired with two controls according to sex and date of birth. Interviews were carried out with the mothers, as well as analysis of medical records. A logistic regression model was used for data analysis following the hierarchical order of entry of the blocks.

Results:

221 live births were allocated in the case group and 442 in the control group. After analysis adjusted for other factors under study, the highest chances of prematurity were associated with being the first child (OR 1.96; 95%CI 1.34–2.86; p=0.001); mothers with the highest income (OR 2.08; 95%CI 1.41–3.08; p<0.001), mothers with previous preterm births (OR 3.98; 95%CI 2.04–7.79; p<0.001), mothers that suffered violence during pregnancy (OR 2.50; 95%CI 1.31–4.78; p=0.005) and underwent cesarean section (OR 2.35; 95%CI 1.63–3.38; p<0.001). Live births to mothers who had more than six prenatal consultations had a lower risk of prematurity (OR 0.39; 95%CI 0.26–0.58; p<0.001).

Conclusions:

The factors associated with a higher chance of prematurity were: higher family income, previous preterm child, primiparity, violence against pregnant women and cesarean section. Having attended more than six prenatal visits was associated with a lower chance of premature birth. Violence against pregnant women showed a strong and consistent association, remaining in all final models, and should serve as an alert for the population and professionals.

Keywords:
Infant, premature; Prenatal care; Risk factors; Pregnancy complications; Violence against women

	Cases (n=221)		Controls (n=442)		OR	95%CI	p-value
	n	%	n	%	OR	95%CI	p-value
Mother’s education							0.048
Complete high school or less	101	45.7	238	53.8	Ref.	–
Complete high school or more	120	54.3	204	46.2	1.39	1.00–1.92
Mother’s age							0.814
20-34 years	149	67.4	302	68.3	Ref.	–
<20 or ≥35 years	72	32.6	140	31.7	1.04	0.74–1.47
Mother’s ethnicity							0.302
White	35	15.8	57	12.9	Ref.	–
Black, brown	186	84.2	385	87.1	0.79	0.50–1.24
Marital status							0.273
No partner	182	82.4	348	78.7	Ref.	–
Partner	39	17.6	94	21.3	0.79	0.52–1.20
Father’s education**							0.077
Complete high school or less	108	53.7	242	61.3	Ref.	–
Complete high school or more	93	46.3	153	38.7	1.36	0.97–1.92
Father’s age**							0.374
20-34 years	138	64.8	295	68.3	Ref.	–
<20 or ≥35 years	75	35.2	137	31.7	1.17	0.83–1.65
Father’s ethnicity							0.586
White	48	21.7	88	19.9	Ref.	–
Black, brown, indigenous	173	78.3	354	80.1	0.90	0.60–1.33
Family’s monthly income **							<0.001
≤2 minimum wages	138	64.2	326	77.6	Ref.	–
>2 minimum wages	77	35.8	94	22.4	1.93	1.35–2.78
Mother’s paid occupation							0.606
No paid occupation	139	62.9	287	64.9	Ref.	–
Paid occupation	82	37.1	155	35.1	1.09	0.78–1.53
Sanitation							1.000
No	27	12.2	54	12.2	Ref.	–
Yes	194	87.8	388	87.8	1.00	0.61–1.64
Residence							0.330
Urban area	190	86.0	367	83.0	Ref.	–
Rural area	31	14.0	75	17.0	0.80	0.51–1.26
Water consumed during pregnancy							0.171
Mineral water, mine, well, cistern or WSS of unaffected municipalities	177	80.1	333	75.3	Ref.	–
WSS of affected municipalities	44	19.9	109	24.7	0.76	0.51–1.13

	Cases (n=221)		Controls (n=442)		OR	95%CI	p-value
	n	%	n	%	OR	95%CI	p-value
Block 2
Interpartum interval							0.228
>2 years	81	36.7	166	37.6	Ref.	–
≤2 years	26	11.8	72	16.3	0.74	0.44–1.25	0.258
First child	114	51.6	204	46.2	1.14	0.81–1.63	0.449
Previous miscarriage							0.465
No	180	81.4	370	83.7	Ref.	–
Yes	41	18.6	72	16.3	1.17	0.77–1.79
Previous stillbirth							1.000
No	217	98.2	434	98.2	Ref.	–
Yes	4	1.8	8	1.8	1.00	0.30–3.36
Child mortality							1.000
No	216	97.7	432	97.7	Ref.	–
Yes	5	2.3	10	2.3	1.00	0.34–2.96
Previous premature child							<0.001
Previous non-premature child	80	36.2	217	49.1	Ref.	–
Previous premature child	27	12.2	21	4.8	3.49	1.87–6.52	<0.001
First child	114	51.6	204	46.2	1.52	1.08–2.14	0.018
Previous low-birth-weight child							0.007
Previous non-low-birth-weight child	86	38.9	218	49.3	Ref.	–
Previous low-birth-weight child	21	9.5	20	4.5	2.66	1.37–5.16	0.004
First child	114	51.6	204	46.2	1.42	1.01–1.99	0.044
Block 3
Alcohol addiction							0.449
No	208	94.1	422	95.5	Ref.	–
Yes	13	5.9	20	4.5	1.32	0.64–2.70
Smoking							1.000
No	205	92.8	410	92.8	Ref.	–
Yes	16	7.2	32	7.2	1.00	0.54–1.87
Drug use							0.314
No	217	98.2	438	99.1	Ref.	–
Yes	4	1.8	4	0.9	2.02	0.50–8.15
Victim of violence during pregnancy							0.004
No	195	88.2	418	94.6	Ref.	–
Yes	26	11.8	24	5.4	2.32	1.30–4.15

	Cases (n=221)		Controls (n=442)		OR	95%CI	p-value
	n	%	n	%	OR	95%CI	p-value
Number of prenatal appointments**							<0.001
<6 appointments	79	36.6	90	20.5	Ref.	–
≥6 appointments	137	63.4	348	79.5	0.45	0.31–0.64
Start of prenatal care**							0.053
≤16 weeks	186	89.4	364	83.7	Ref.	–
>16 weeks	22	10.6	71	16.3	0.61	0.36–1.01
Prenatal care network**							0.014
Public network	166	77.9	376	85.6	Ref.	–
Private network	47	22.1	63	14.4	1.69	1.11–2.57
Vaccination during pregnancy							<0.001
No	23	10.4	15	3.4	Ref.	–
Yes/immunized	198	89.6	427	96.6	0.30	0.15–0.59
Residence covered by FHS							0.125
No	17	7.7	21	4.8	Ref.	–
Yes	204	92.3	421	95.2	0.60	0.31–1.16
Type of delivery							<0.001
Natural	104	47.1	295	66.7	Ref.	–
Cesarian	117	52.9	147	33.3	2.26	1.62–3.14

Blocks		OR	95%CI	p-value
Block 1	Monthly family income			<0.001
	≤2 minimum wages	Ref.	–
	>2 minimum wages	2.08	1.41–3.08
Block 2	Previous premature child			<0.001
	Previous non-premature child	Ref.	–
	Previous premature child	3.98	2.04–7.79	<0.001
	First child	1.96	1.34–2.86	0.001
Block 3	Victim of violence during pregnancy			0.005
	No	Ref.	–
	Yes	2.50	1.31–4.78
Block 4	Number of prenatal appointments			<0.001
	<6 appointments	Ref.	–
	≥6 appointments	0.39	0.26–0.58
	Type of delivery			<0.001
	Natural	Ref.	–
	Cesarian	2.35	1.63–3.38

Sociedade de Pediatria de São Paulo R. Maria Figueiredo, 595 - 10o andar, 04002-003 São Paulo - SP - Brasil, Tel./Fax: (11 55) 3284-0308; 3289-9809; 3284-0051 - São Paulo - SP - Brazil
E-mail: rpp@spsp.org.br

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[1] *Corresponding author. E-mail: erica.defilipo@ufjf.edu.br (E. C. Defilipo).