Moderate Preterm Birth Research Articles

A Prospective Population-Based Study of Birth at 32-36 Weeks of Gestation: Neonatal Outcomes from the Late and Moderate Preterm Birth Study (LAMBS)

Background: Although late and moderate prematurity (32-36 weeks' gestation) represents 75% of preterm births, research in these infants is sparse. However, data from small or retrospective studies consistently suggest they are at greater risk of adverse outcomes than previously recognised.Aim: A prospective population-based study of outcomes of infants born at 32-36 weeks' gestation.Methods: All mothers and their babies delivering at 32-36 weeks were invited to participate, together with a random sample of infants born at ≥37 weeks. Mothers were interviewed and obstetric and neonatal records were reviewed.Results: Preliminary data analysis for 728 preterm and 782 term live-born singleton infants recruited between September 2009 and September 2010 revealed that neonatal outcomes for preterm infants were significantly worse than those of term born infants (Table 1).Table 1 Full size tableConclusions: Births at 32-36 weeks far outnumber those below 32 weeks. The overall impact of adverse outcomes in these children is therefore likely to be greater than that of the most immature infants. This has implications for health services planning and resource allocation.

Klinische und ökonomische Herausforderung der moderaten Frühgeburt zwischen 32+0 und 36+6 SSW

Preterm births show a worldwide increasing incidence. The majority of preterm births occur between 32+0 and 36+6 weeks of gestation and are associated with an increased rate of maternal and neonatal morbidity. The focus of our research is the clinical and economical analysis of all preterm births between 32+0 and 36+6 weeks of gestation in a German level 1 perinatal centre over a period of 3 years. A retrospective analysis of all preterm births between 32+0 and 36+6 weeks of gestation in the University Hospital Mainz from 2007 to 2009 was undertaken. Data were collected using our electronic documentation system. Gestational age at delivery, mode of birth, indication for delivery, duration of the peripartum treatment, treatment of the newborn in the children's hospital, birth weight and therapy costs were evaluated. We recorded 407 moderate preterm births in total; this amounts to a rate of of 10% of all births. Major causes of prematurity were PPROM, preterm labour and preeclampsia/HELLP. Maternal and fetal systemic diseases were more uncommon. Rates of Caesarean sections (62%) and of neonatal inpatient treatment needs (58.5%) were high. Maternal treatment costs were 332 Euro/day. The mean duration of maternal inpatient treatment was 13.15 days. Moderate preterm birth is associated with maternal morbidity frequently due to a high rate of Caesarean sections. Neonatal morbidity is also increased. In comparison with previous research, we saw an increased rate of pregnancy complications. This could be typical for a level 1 perinatal centre. Moderate preterm birth is seen as the cause of considerable treatment costs.

Preterm Birth and ADHD in School Children

Researchers at Karolinska Institutet, Sachs Children’s Hospital, Stockholm, and Uppsala and Stockholm Universities, Sweden, analyzed the effect of moderate and extreme preterm birth on the risk for ADHD in school age children who have survived from NICUs, and allowing for genetic, perinatal, and socioeconomic confounders.

Rising Disparities in Severe Adverse Birth Outcomes Among Haitians in Québec, Canada, 1981–2006

Perinatal health data for Haitians are scant. We evaluated adverse birth outcomes for Haitians in Québec, Canada. We analyzed 2,124,909 live births from 1981 to 2006. Haitian ethnicity was assessed using maternal birth country (Haiti, other Caribbean country, other foreign country, Canada) and home language (Creole, French/English but Creole mother tongue, French/English, other). Associations between ethnicity and preterm birth (PTB), low birth weight (LBW), and small-for-gestational-age (SGA) birth were calculated. Adverse birth outcomes were more common among mothers with Haitian ethnicity. Relative to Canadian-born mothers, odds for Haitian-born mothers were 4 times greater for extreme PTB (≤27 weeks), twice greater for very PTB (28-31 weeks), and 25% higher for moderate PTB (32-36 weeks). Patterns were similar for SGA birth and severe cases of LBW. Despite overall decreases LBW and SGA birth, relative and absolute inequalities increased over time. Perinatal health inequalities are increasing for Haitian-born mothers.

956 Born at 32-36 Weeks - Is this a Problem?

Aim: To describe morbidity and mortality for infants born with GA 32-36 weeks in a Danish County. Material and Methods: Included were all infants with GA 32-36 weeks, born 2000-2006 and admitted to the neonatal unit at Hospital Lillebaelt. Data is from “Neobase” with data on treatment for all admissions. Further data on survival by the end of 2009 was included. Total births in the county were 32339. Results: The study included 1252 infants. Prevalence of births 32-36 weeks was 39 pr 1000 births. 447 infants (36 %) were from a multiple pregnancy. Twenty infants (1.6 %) died, 14 within the first week, 3 within 40 days and 3 within 6 months. The majority of infants (17, 85%) died with major congenital malformations. More than half of the infants were delivered by Caesarean section (640, 51%). 465 infants (37%) were treated with CPAP and 346 for more than 1 day. Fifty-one infants (4%) received surfactant treatment and 6 of these died. 467 infants (37%) received treatment for hypoglycemia and 289 needed glucose infusion. NEC was suspected clinically in only 1 infant. Conclusions: 3.9 % of all newborns in the population were admitted to the neonatal unit for treatment for moderate preterm birth. Infants born within GA 32-36 often need respiratory support. Mortality is much lower than for infants born with GA < 32 weeks, where a similar study from the “Neobase” showed a mortality of 11%, but higher than for term born infants. Mortality is mainly due to congenital malformations.

Increasing educational inequality in preterm birth in Québec, Canada, 1981–2006

BackgroundFew studies have evaluated the relationship between preterm birth (PTB) and maternal education over time. We sought to determine whether educational inequalities in PTB have increased in Québec, Canada.MethodsThe authors...

Early or Recurrent Preterm Birth and Maternal Cardiovascular Disease Risk

Preterm birth (PTB) has been associated with a later increased risk of maternal cardiovascular disease (CVD). We hypothesized a more pronounced relation between early or recurrent PTB and maternal CVD risk. We related PTB severity (earlier gestational age at delivery) and recurrence (>/=2) among women with births from 1973-1983 in Denmark (n = 427,765) to maternal CVD morbidity or mortality (1977-2006). Birth data were linked to CVD hospitalizations and deaths identified in national registers and data were analyzed using Cox proportional hazards models. Women with a prior PTB had excess CVD after adjustment for age, parity, and education (hazard ratio [HR] = 1.36 [95% confidence interval (CI): 1.31, 1.41]). This was only modestly attenuated when women with preeclampsia or small for gestational age births were excluded, and the relationship was stronger for CVD mortality (HR = 1.98 [1.73, 2.26]). Recurrent PTB was associated with higher CVD morbidity compared to women with one PTB, particularly for ischemic events (HR = 1.78 [1.40, 2.27] vs. 1.22 [1.09, 1.36]). Risk was similarly elevated among women with early, moderate, and late PTB. Sensitivity analysis suggested that confounding by smoking only partly explained these associations. Women with PTB, especially recurrent PTB, were at increased risk for CVD, suggesting common causes of these conditions.

Neonatal Outcomes in Twin Pregnancies Delivered Moderately Preterm, Late Preterm, and Term

We compared neonatal outcomes in twin pregnancies following moderately preterm birth (MPTB), late preterm birth (LPTB), and term birth. A secondary analysis of a multicenter, randomized controlled trial of multiple gestations was conducted. MPTB was defined as delivery between 32 (0)/(7) and 33 (6)/(7) weeks and LPTB between 34 (0)/(7) and 36 (6)/(7) weeks. Primary outcome was a neonatal outcome composite consisting of one or more of the following: neonatal death, respiratory distress syndrome, early onset culture-proven sepsis, stage 2 or 3 necrotizing enterocolitis, bronchopulmonary dysplasia, grade 3 or 4 intraventricular hemorrhage, periventricular leukomalacia, pneumonia, or severe retinopathy of prematurity. Among 552 twin pregnancies, the MPTB rate was 14.5%, LPTB 49.8%, and term birth rate 35.7%. The rate of the primary outcome was different between groups: 30.0% for MPTB, 12.8% for LPTB, 0.5% for term birth ( P < 0.001). Compared with term neonates, the primary neonatal outcome composite was increased following MPTB (relative risk [RR] 58.5; 95% confidence interval [CI] 11.3 to 1693.0) and LPTB (RR 24.9; 95% CI 4.8 to 732.2). Twin pregnancies born moderately and late preterm encounter higher rates of neonatal morbidities compared with twins born at term.

Maternal BMI and preterm birth: A systematic review of the literature with meta-analysis

Objectives. To examine the association between high prepregnancy maternal body mass index (BMI) and the risk of preterm birth (PTB).Methods. A systematic review of the literature. We included cohorts and case-control studies published since 1968 that examined the association between BMI and PTB of all types, spontaneous (s), elective and with ruptured membranes (PPROM) in three gestational age categories: general (<37 weeks), moderate (32–36 weeks) and very (<32 weeks) PTB.Results. 20,401 citations were screened and 39 studies (1,788,633 women) were included. Preobese (BMI, 25–29.9) and obese I (BMI, 30–34.9) women have a reduced risk for sPTB: AOR = 0.85 (95% CI: 0.80–0.92) and 0.83 (95% CI: 0.75–0.92), respectively. Their risk for moderate PTB was 1.20 (95% CI: 1.04–1.38) and 1.60 (95% CI: 1.32–1.94), respectively. Obese II women (BMI, 35–40) have an increased risk for PTB in general (AOR = 1.33, 95% CI: 1.12–1.57) moderate (AOR = 2.43, 95% CI: 1.46–4.05) and very PTB (AOR = 1.96, 95% CI: 1.66–2.31). Obese III women (BMI > 40) have an even higher risk for very PTB (AOR = 2.27, 95%CI: 1.76–2.94). High BMI does not modify the risk for PPROM and increases the risk for elective PTB.Conclusions. High maternal BMI may have different effects on different types of PTB.

Cervical length and obstetric history predict spontaneous preterm birth: development and validation of a model to provide individualized risk assessment

To evaluate the ability of combinations of cervical length and maternal history to assess the risk of spontaneous preterm birth, and to provide a simple procedure for the optimal estimation of risk. This prospective observational study was carried out between January 1998 and May 2006. Transvaginal sonographic measurement of cervical length at 20 + 0 to 24 + 6 weeks of gestation was carried out in 58 807 singleton pregnancies as part of routine antenatal care. The outcome measure was spontaneous extreme (< 28 weeks), early (28-30 weeks), moderate (31-33 weeks) and mild (34-36 weeks) preterm birth. Logistic regression analysis was used to derive models for the prediction of spontaneous preterm birth from the maternal obstetric history, demographic characteristics and cervical length. The rates of extreme, early, moderate and mild spontaneous preterm birth were 0.23%, 0.24%, 0.57% and 2.93%, respectively. The best prediction of spontaneous preterm birth was provided by cervical length (area under the receiver-operating characteristics curve (AUC), extreme 0.903, early 0.816, moderate 0.784 and mild 0.617) and this was improved by adding obstetric history (AUC, extreme 0.919, early 0.836, moderate 0.819 and mild 0.650). Addition of other parameters was without material effect. For a 10% screen-positive rate, models using cervical length and obstetric history had a sensitivity of 80.6%, 58.5%, 53.0% and 28.6% for extreme, early, moderate and mild spontaneous preterm birth, respectively. These models were expressed as tables of adjusted likelihood ratios to allow simple estimation of the risk of spontaneous preterm birth. A model combining cervical length and obstetric history provides a better prediction of spontaneous preterm birth than either factor alone, and the sensitivity of screening improves for increasing degrees of prematurity.

Early pregnancy lipid concentrations and spontaneous preterm birth

Women who deliver preterm infants may be at increased risk for cardiovascular disease, perhaps related to dyslipidemia. In a nested case control study of women with spontaneous preterm birth, cholesterol, high-density lipoprotein, low-density lipoprotein, and triglycerides were evaluated. Lipid concentrations and gestational changes, as well as risk for preterm birth, were evaluated in women who delivered <34 (n = 23), >or=34-<37 (n = 67), and >or=37 weeks (n = 199). High cholesterol or triglycerides <or=15 weeks were associated with a 2.8-fold (1.0-7.9) and 2.0-fold (1.0-3.9) increased risk for preterm birth <34 weeks and >or=34-<37 weeks, respectively. Overweight women who delivered <34 weeks had particularly elevated early pregnancy concentrations of cholesterol and low-density lipoprotein; lean women with moderate preterm birth had elevated triglycerides. There was a reduced triglyceride response in the first half of pregnancy among women who delivered <34 weeks. Our results indicate the presence of dyslipidemia in women with spontaneous preterm birth.

Lung Parenchyma at Maturity Is Influenced by Postnatal Growth but Not by Moderate Preterm Birth in Sheep

Background: We have recently shown that moderate preterm birth, in the absence of respiratory support, altered the structure of lung parenchyma in young lambs, but the long-term effects are unknown. Objectives: To determine whether structural changes persist to maturity, and whether postnatal growth affects lung structure at maturity in sheep. Methods: At approximately 1.2 years after birth, lung parenchyma of sheep born 14 days before term (n = 7) was stereologically compared with that of controls born at term (n = 8, term approx. 146 days). Results: Preterm birth per se had no significant effect on lung volume, alveolar number and size, and thicknesses of the alveolar walls and blood-gas barrier. After combining the preterm and term groups, we examined the effects of postnatal growth rates on lung parenchyma. Slower-growing sheep (SG; n = 7: 4 preterm, 3 term) were compared with faster-growing sheep (FG; n = 8: 3 preterm, 5 term). At approximately 1.2 years, the right lung volume, relative to body weight, was significantly lower in SG than FG sheep (p < 0.05) and alveolar number was significantly lower by approximately 44%. The total alveolar internal surface area of the right lung of SG sheep was 38% smaller than in FG sheep; it was also significantly lower when related to both lung and body weight. Conclusions: Our data suggest that moderate preterm birth does not cause persistent alterations in lung parenchyma. However, slow postnatal growth in low-birth-weight sheep results in smaller lungs with fewer alveoli and a lower alveolar surface area relative to body weight.

Preterm Infants as Young Adults: A Swedish National Cohort Study

Increasing numbers of infants born preterm survive into adulthood. In this study, we analyzed the effect of having been born preterm on disability and vocational success in young adults. A Swedish national cohort of 522,310 infants born in 1973-1979 were followed up for disabilities and income in national registers in 2002 at the age of 23 to 29. Hypotheses were tested in multivariate analysis with logistic regression models on the log scale for dichotomized outcomes and linear regression for continuous variables. There was a stepwise increase in disability in young adulthood with increasing degree of preterm birth. A total of 13.2% of children born at 24 to 28 weeks' gestation and 5.6% born at 29 to 32 weeks' gestation received economic assistance from society because of handicap or persistent illness, which is equivalent to nearly 4 [corrected] times the risk of those born at term after adjustment for socioeconomic and perinatal confounders. Moderate (33-36 weeks' gestation) and marginal (37-38 weeks' gestation) preterm birth also carried significantly increased risks for disability and were responsible for 74% of the total disability associated with preterm birth. Preterm birth was associated with a lower chance of completing a university education and a lower net salary in a stepwise manner. The total economic gain for Swedish society, in terms of taxes and decreased costs for benefits, if all long-term effects of preterm birth could have been prevented in the birth cohorts in this study, would have amounted to 65 million euros in 2002 alone. The majority of adults who were born very preterm lived an independent and self-supportive life. Moderately preterm birth carries a considerable risk for long-term impairment. There are strong economic incentives for secondary prevention of disability associated with preterm birth.

Circulating Angiogenic Factors in Early Pregnancy and the Risk of Preeclampsia, Intrauterine Growth Restriction, Spontaneous Preterm Birth, and Stillbirth

To estimate the relationship between maternal serum levels of placental growth factor (PlGF) and soluble fms-like tyrosine kinase-1 (sFlt-1) in early pregnancy with the risk of subsequent adverse outcome. A nested, case-control study was performed within a prospective cohort study of Down syndrome screening. Maternal serum levels of sFlt-1 and PlGF at 10-14 weeks of gestation were compared between 939 women with complicated pregnancies and 937 controls. Associations were quantified as the odds ratio for a one decile increase in the corrected level of the analyte. Higher levels of sFlt-1 were not associated with the risk of preeclampsia but were associated with a reduced risk of delivery of a small for gestational age infant (odds ratio [OR] 0.92, 95% confidence interval [CI] 0.88-0.96), extreme (24-32 weeks) spontaneous preterm birth (OR 0.90, 95% CI 0.83-0.99), moderate (33-36 weeks) spontaneous preterm birth (OR 0.93, 95% CI 0.88-0.98), and stillbirth associated with abruption or growth restriction (OR 0.77, 95% CI 0.61-0.95). Higher levels of PlGF were associated with a reduced risk of preeclampsia (OR 0.95, 95% CI 0.90-0.99) and delivery of a small for gestational age infant (OR 0.95, 95% CI 0.91-0.99). Associations were minimally affected by adjustment for maternal characteristics. Higher early pregnancy levels of sFlt-1 and PlGF were associated with a decreased risk of adverse perinatal outcome.

Gestational age shortening in single births at term. Italy 1990–1998

The increasing incidence of moderate preterm births (32-36 gestation weeks) might reflect a more general tendency toward a shortening of the gestational length occurring also in the term births (37 + gestation weeks). We examined all Italian 1st live born singletons (n = 2,356,365) and found that from 1990 to 1998 the births of 40 + gestation weeks decreased from 60.7 to 51.7% and among term births the average gestation weeks decreased from 39.74 to 39.55. In term pregnancies the effect of low education and advanced age of the mother in decreasing the gestational length persisted over time, but, independently of the maternal factors, the pregnancies experienced a progressive shortening. The finding, if confirmed for other countries, should deserve further investigations on possible determinants, as improved estimates of gestational age through the widespread use of prenatal ultrasound or diffusion of hazardous and stressing working and living conditions.

Paternal Age and Preterm Birth

The authors respond: With regard to our paper1 on the impact of paternal age on very preterm (<32 weeks’ gestation) and moderate preterm (32–36 weeks’ gestation) births, Helen Dolk2 wonders whether in our data, routinely collected from the National Institute of Statistics (ISTAT), the reported parental age suffers from errors that possibly affect our results. We are happy to reply with regard to 1) data quality, 2) confounding effect of maternal age, and 3) reproducibility of our results. 1) Gross errors are routinely checked by ISTAT. Our analysis sample included only Italian babies; mothers from other countries, who may report less reliable information, were excluded. We also limited our analysis to babies, with both parents age 20+ years, and for whom complete information had been recorded. Thus, potential errors associated with maternity in single mothers or in teenagers are reduced. 2) We agree that, in case of misreported paternal age, the effect could be underestimated or, alternatively, it could be overestimated if, in couples with a large paternal–maternal age gap, older mothers are classified as young. Unfounded results could come from mothers age 30+ years, who because they underreport their own age, are registered in the age class of 20–24 years and thus inflate the effect of paternal age. Figure 1 shows that the risk of preterm births stays about the same for maternal age 20 to 24 years, increases slightly from 25 to 29 years, and increases rapidly only from 30 years onward. Although we cannot exclude maternal age underclassification, it is unlikely that this would account for more than a 5-year error in a large number of couples.FIGURE 1.: Very preterm (<32 weeks’ gestation) and moderate preterm (32–36 weeks’ gestation) births by maternal age in Italy, 1990–1998.Furthermore, most of the maternal-age effect was removed by limiting maternal age to 20–24 and 25–29 years and by adjustment for the intraclass effect of maternal age in single years.1 3) The opportunity to analyze the 1971–1979 and 1982–1989 data sets allows us to confirm our findings on the 1990–1998 data.1 We find that, for these years, as well, among mothers age 20–24 years, a wide paternal–maternal age gap increases the risk of preterm births (Table 1). The odds ratios (and associated 95% confidence intervals) estimated through logistic regression models, adjusted for education, year, and sex, confirm a stronger effect on very preterm than on moderate preterm births.TABLE 1: Association of Very Preterm (<32 Weeks’ Gestation) and Moderate Preterm (32–36 Weeks’ Gestation) Births With Paternal Age, in Mothers of 20–24 Years, in Italy, 1971–1979 and 1982–1989Adverse socioeconomic conditions in “resourceless” young mothers, with little education, and old fathers are likely responsible of the patterns. However, these findings of an effect persisting over time support a potential role of spontaneous or induced genetic mutations as well. Paola Astolfi Department of Genetics and Microbiology, A. Buzzati-Traverso, University of Pavia, Pavia, Italy, [email protected] Antonio De Pasquale Laura A. Zonta

Paternal Age and Preterm Birth in Italy, 1990 to 1998

Advanced paternal age has been reported to impair pregnancy outcome. Here, we investigated the association of advanced paternal age with preterm birth by using a very large national data set. We analyzed data from 1990 to 1998 on Italian firstborn singletons to mothers 20-24 and 25-29 years of age (n = 1,510,823). Odds ratios for overall preterm (<37 weeks' gestation), very preterm (<32 weeks), and moderate preterm (32-36 weeks) births were evaluated through logistic regression models in paternal age classes (20-24, 25-29, 30-34, 35-39, 40-44, 45-49, 50+ years) after adjustment for confounders. Nonparametric regression models were used to fit the effect of paternal ageing on the incidence of very preterm births. Odds ratios increased with paternal age more rapidly for very preterm than for moderate preterm births; among 45- to 49-year-old fathers, odds ratios for very preterm births reached 1.91 (95% confidence interval = 1.08-3.38) and 1.72 (1.25-2.36), respectively, in 20- to 24- and 25- to 29-year-old mothers. This study confirms that paternal age contributes to the risk of preterm birth. The effect is stronger on very preterm births but also influences moderate preterm births.

Increasing pre-term and low-birth-weight rates over time and their impact on infant mortality in south-east Brazil.

This study investigates the possible effects of pre-term births and low birth weight on infant mortality rates (IMRs) over a 15-year period in Ribeirão Preto, Brazil, based on surveys carried out in 1978/79 and 1994. The 1978/79 survey included 6750 births over a 12-month period and the 1994 survey 2846 births over a 4-month period. Infant deaths were retrieved monthly from the city register. Infant mortality rate decreased from 36.6 to 16.9 deaths per 1000 over 15 years. The decrease in IMR was larger in the 2500-2999 g group than in any other group. The observed falls in IMR were attributable to decreases in birth-weight-specific mortality rates. Likewise, there was a general decrease in IMR in mild, moderate and severe pre-term births. The incidence rate ratio of infant mortality between surveys was 0.46 (95% CI 0.34-0.63); it increased to 0.57 (95% CI 0.35-0.75) when adjusted for birth weight and other factors in the model and rose to 0.69 (95% CI 0.49-0.97) when adjusted for length of gestation and other variables. The increase in pre-term births and low birth weight may have had, at most, a marginal effect on the IMR. Progress in the care of newborns may have decreased the mortality risk, but even mild pre-term birth still has an impact on infant mortality. There is room for further improvement in IMR by tackling the high rates of pre-term birth.

Physical load and psychological demand at work during pregnancy and preterm birth.

To evaluate the effect of physical workload and psychological demand on all preterm births, and to determine whether these risk factors have the same effect on different types of preterm birth (moderate versus very preterm birth) and different modes of delivery onset (spontaneous versus indicated preterm birth). A case-control study was carried out in two public general hospitals in the Valencia Region, Spain. All preterm births (228) which occurred between 22 and 36 completed weeks of amenorrhea and 348 controls of 37 or more completed weeks of amenorrhea were included. The information was collected by interviewing women within 2 days of their giving birth. Physical workload, psychological demand, weekly working hours and daily time spent commuting between home and work were used as explanatory variables. A polytomous logistic regression was carried out. Exposure to medium or high level physical workload increases the risk of preterm birth, with an adjusted odds ratio (OR) of 1.59 and 2.31, respectively. The risk of moderate preterm birth was greater in women with a medium or high level of physical workload, OR: 1.73 and 2.35, respectively. The same trend was observed for very preterm birth. Physical workload showed a different effect on spontaneous and indicated preterm birth. The exposure to medium and high level physical workload increases the risk of indicated preterm birth, with an OR of 2.74 and 3.88, respectively. The same trend was seen in the case of spontaneous preterm birth. Psychological demands were not associated with preterm birth. High physical exertion increases the risk of preterm birth in Spain. The magnitude of the effect of physical workload on moderate and very preterm birth is similar, but is higher on indicated preterm birth than on spontaneous preterm birth. Psychological demands show no effect on the risk of preterm birth.

Factores socioeconómicos asociados al parto pretérmino. Resultados del proyecto EUROPOP en el Estado español

The objectives of this paper were to analyse the effect of social, personal and medical risk factors on preterm birth (moderate versus very preterm) or on two preterm birth groups (spontaneous versus indicated). Results from the Spanish collaborating centre of the European multicentre case-control study EUROPOP (European Program of Occupational Risks and Pregnancy Outcome) are presented. All preterm births (529) between 22 and 36 completed weeks of amenorrhea and 788 births of 37 or more completed weeks of amenorrhea (control group) are included. Explicative variables are divided in social, personal and medical factors. A univariate and multivariate analysis by means of a logistic regression were carried out. Very preterm birth risk was higher for women over 34 years, adjusted OR: 2.53 (1.42-4.52), with lower educational level, adjusted OR: 1.79 (1.07-2.98), for primigravid women or multigravid women with only first trimestre abortion, adjusted OR: 1.86 (1.13-3.04), and for multigravid women with previous preterm birth or second trimestre abortion, adjusted OR: 5.53 (2.97-10.35). A similar trend was observed for moderate preterm birth. Probability of spontaneous preterm birth was higher for mother over 34 years, adjusted OR: 1.51 (1.01-2.26), with lower income, adjusted OR: 1.75 (1.07-2.88) and for multigravid women with previous preterm birth or second trimestre abortion, adjusted OR: 2.96 (1.86-4.71). Results were similar for indicated preterm birth. Social differences were found to be related to moderate and very preterm birth. No differences were observed between risk factors and kind of preterm birth: spontaneous or indicated.