Maternal risk factors for preterm birth: a country-based population analysis

Abstract

Objective

The aim of this study was to identify maternal risk factors for spontaneous preterm birth (PTB) compared to delivery at term, in order to recognize high risk women and to provide a global overview of the Italian situation.

Study design

A multicenter, observational and retrospective, cross-sectional study was designed. The study population comprised 7634 women recruited in 9 different University Maternity Hospitals in Italy. The main criteria for inclusion were: women having had vaginal preterm or term spontaneous delivery in each participating centre during the study period. The records related to deliveries occurring between April and December 2008. A multivariable logistic regression was employed to identify independent predictors of spontaneous preterm birth. Odds ratios (ORs) and 95% confidence intervals (95% CI) were reported with two-tailed probability (p) values. Statistical calculations were carried out using SAS version 9.1. A two-tailed p-value of 0.05 was used to define statistical significant results.

Results

A significant increased risk of PTB was found in women with BMI > 25 (OR = 1.662; 95% CI = 1.033–2.676; p-value = 0.0365) and in women employed in heavy work (OR = 1.947; 95% CI = 1.182–3.207; p-value = 0.0089). Moreover there was a significant association between PTB and previous reproductive history. In fact a history of previous abortion (OR = 1.954; 95% CI = 1.162–3.285; p-value = 0.0116) or previous cesarean section (OR = 2.904; 95% CI = 1.066–7.910; p-value = 0.0371) was positively correlated to the increased risk of PTB and an important statistically significant association was calculated between PTB and previous pre-term delivery (OR = 3.412; 95% CI = 1.342–8.676; p-value = 0.0099). All the other covariates examined as potential risk factors for PTB were not found to be statistically significantly related (p-value > 0.05).

Conclusions

The present study, applied to a substantial sample of Italian population, demonstrates that there are peculiar risk factors for spontaneous PTB in the Italian population examined. It shows an association between preterm delivery and certain maternal factors as: BMI, employment, previous abortions, previous PTBs and previous cesarean section.

Introduction

Pre-term birth (PTB) is defined as birth before 37 completed weeks of gestation. Its incidence can vary depending on the geographical and demographic features of the population studied, and in Italy PTB comprises 6–7% of all births [1]. The precise role of events linked to an increased risk of PTB is largely unknown, but decidual hemorrhage, mechanical factors, hormonal changes and cervicovaginal infections have been associated with the pathophysiology of PTB [2], [3]. The incidence of PTB is rapidly growing, partly because today it includes some cases that in the past resulted in second trimester “late” abortions. The increase in industrialized countries is strongly linked to the rise of maternal age [4] and of the number of pregnancies following assisted reproductive treatments [5] as well as to the introduction of new risk factors related to life style [6].

It is nowadays important to investigate epidemiological and environmental factors to identify high risk women. The risk of PTB first of all correlates to intrinsic characteristics of the mother; it is different between racial and ethnic groups [7], [8] and is related to advanced maternal age [4]. In order to identify women at risk it is also important to assess maternal life style, level of education [6] and adequacy of prenatal care [9], [10], as well as employment-related psychological and physical stress [11], [12], [13]. Furthermore maternal BMI [14], [15], nutritional status, chronic diseases (such as hypertension, diabetes mellitus) [16], intrauterine malformation or infections, and endocrinological diseases have been linked with an increased risk of PTB.There is consistent evidence that having a history of PTB is the strongest predicting factor [17]. Previous abortion, independent of the type (spontaneous or induced), increases the possibility of PTB [18], [19]; moreover there is an increasing risk of very preterm birth associated with increasing numbers of abortions [20]. Pregnancies resulting from assisted reproductive treatment (IVF, ICSI) can increase the risk of PTB [5]. Similarly even an invasive diagnostic tool, such as amniocentesis or villocentesis, is demonstrated to be a potential risk factor [21], [22].

On the basis of these data it is clear that etiology of PTB is multi-factorial, and numerous exposures including social, psychological, biological and genetic factors are known to be associated. Results of studies coming from the “western world” not always are applicable from one situation to another. Nowadays in these countries there is also a lack of studies analyzing specific risk factors, so it would be necessary to make a distinction based on maternal origin and on different lifestyles. The aim of this study was to identify maternal risk factors for spontaneous PTB compared to delivery at term, in order to recognize high risk women and to provide a global overview of the Italian situation. The study sample comes from 9 different University Maternity Hospitals distributed throughout the country.