Social differences of very preterm birth in Europe: interaction with obstetric history. Europop Group.

Social differences of very preterm birth (22-32 completed weeks of amenorrhea) were studied using data from a large case-control survey in Europe between 1994 and 1997; 1,675 very preterm births and 7,965 full-term births were included. The relation between social factors and very preterm birth was studied according to obstetric history and the mode of delivery onset. Very preterm birth was significantly related to low educational level among women with no previous adverse pregnancy outcome (odds ratio (OR) = 2.67, 95 percent confidence interval (CI) 1.66-4.28) and among primigravid women and those with previous first-trimester abortion (OR = 2.01, 95 percent CI 1.56-2.58). In this group, unemployment of all household members was associated with a double risk of very preterm birth. No significant association between very preterm birth and socioeconomic status was observed among women with previous second-trimester abortion or preterm birth. Socioeconomic indicators remained significantly associated with both spontaneous and induced very preterm births among women with no previous late fetal loss or preterm birth. The results are consistent with social factors affecting the risk of very preterm birth, but the relation differs according to obstetric history.     

Smoking during pregnancy according to obstetric complications and parity: results of the EUROPOP study

This study aimed to analyse the relationship between smoking and preterm birth (22-36 weeks) according to the main obstetric complications leading to the preterm birth, both overall and by parity. The EUROPOP study is a case-control study carried out between 1994 and 1997; 3,787 preterm and 5,602 full-term births were included, from maternity units in 10 countries, using the same protocol. Social, demographic and medical information was collected after delivery, from obstetric records and interviews with the mothers. Cases were classified according to the main obstetric complication (hypertension, haemorrhage, preterm premature rupture of membranes (PPROM), idiopathic spontaneous preterm labour, intrauterine growth retardation, all other causes). Multiple logistic regression analysis was used to control for confounders. Twenty four percent of cases and 20% of controls were smokers. Smoking during pregnancy, heavy smoking (>or=10 cigarettes per day) in particular, was a risk factor for preterm birth (aOR = 1.39, 95% CI:1.20-1.60). Smoking increased the risk of preterm delivery due to all obstetric complications other than hypertension. For these complications, the risk of preterm delivery associated with smoking was higher for multiparae (aOR = 1.46, 95% CI:1.24-1.71) than for primiparae (aOR = 1.18, 95% CI:1.00-1.38). In conclusion, smoking during pregnancy increases the risk of preterm birth among women with all obstetric complications except hypertension. This association is stronger in multiparae than in primiparae and the risk is higher for heavy smokers.     

Smoking during pregnancy and preterm birth according to obstetric history: French national perinatal surveys

The relation between smoking and preterm delivery is not totally known. Our aim was to determine whether smoking during pregnancy was associated with preterm birth among women at different risk according to their obstetric history. The study was based on data from the 1998 French national perinatal survey. Of the 13073 singleton live births, 4.7% were preterm; 15% of the pregnant women were moderate (one to nine cigarettes per day) and 10% heavy smokers (at least 10 cigarettes per day). Smoking heavily was related to preterm birth (crude odds ratio [OR] = 1.35, 95% confidence interval [95% CI]: [1.04, 1.74]). Multivariable logistic regression showed a relation between smoking and preterm birth among multiparae without previous adverse pregnancy outcomes; the associated adjusted ORs (AORs) were 1.25 [95% CI 0.83, 1.87] among moderate smokers and 1.46 [95% CI 0.98, 2.20] among heavy smokers. The corresponding AORs were 0.69 [95% CI 0.46, 1.05]) and 0.96 [95% CI 0.59, 1.56] for primiparae and 1.11 [95% CI 0.63, 1.93] and 0.50 [95% CI 0.25, 0.98] for multiparae with previous adverse pregnancy outcomes. Our study showed a relation between heavy smoking during pregnancy and preterm birth mostly for women with low obstetric risk.     

兰州地区早产影响因素的研究

目的 探讨早产、自发性早产和医源性早产的相关高危因素.方法 回顾性调查2010年1月至2012年12月在甘肃省妇幼保健院住院分娩足月儿和早产儿的孕妇的住院资料,分娩足月活产儿5639例、早产儿540例,分为足月产组(≥37周孕龄)和早产组(<37周孕龄),比较早产组和足月产组发生早产的相关危险因素,以及自发性早产和医源性早产的相关高危因素.结果 ①Logistic回归分析发现教育年限(≥16年)(OR=0.61,95%CI:0.48~0.78)、家庭平均月收入(>3000元)(OR=0.62,95%CI:0.50~0.78)是早产发生的保护因素,而母亲妊娠合并症,比如妊娠期糖尿病(GDM)(OR=3.97,95%CI:1.70~9.25)、妊娠期高血压疾病(HDP)(OR=4.43,95%CI:3.35~5.87)、妊娠期胆汁淤积症(ICP)(OR=4.88,95%CI:3.25~7.32)是早产发生的独立高危因素.②按早产的病因分类,本研究中340例为自发性早产儿,余200例为医源性早产.经分层多因素非条件Logistic回归分析表明,与足月产相比,经产妇(OR=2.66,95%CI:1.87~3.76)、GDM(OR=4.52,95%CI:1.42~14.38),尤其是HDP孕妇(OR=14.19,95%CI:10.10~19.93)更易发生医源性早产,而ICP孕妇更多出现自发性流产(OR=12.875,95%CI:12.75~13.00).结论 应及早识别早产潜在的高危因素,加强围生期管理,以减少早产的发生,改善围生儿结局.     

The influence of maternal age on very preterm birth of twins: differential effects by parity

For singleton births, parity can modify the effect of maternal age on birth outcomes such as low birthweight and preterm birth; however, it is unknown whether this relationship exists for twin births. As the rate of twin births increases among older women, it is important to understand how parity may influence the relationship between maternal age and adverse birth outcomes. The NCHS Matched Multiple Birth Data Set, which contains all twin births in the USA from 1995 to 1998, was analysed. Parity was grouped into two levels (primiparous--no prior live births, and multiparous--at least one prior live birth), and maternal age was divided into the following groups: 20-24, 25-29, 30-34, 35-39, and 40 years or more. Very preterm birth was defined as births occurring before 33 weeks. Logistic regression was used to obtain odds ratios (OR) to estimate the risk of very preterm birth, and to determine the relationships between parity, maternal age, and very preterm birth. Among primiparae, women 40 years and older had a reduced risk of very preterm birth compared with women of 25-29 years (OR 0.74 [95% CI=0.66, 0.84]). Among multiparae, women 40 years and older had the same risk of very preterm birth compared with women of 25-29 years (OR 1.00 [95% CI=0.90, 1.12]). However, stratification by education revealed that the age gradient was limited to women with >12 years education among primiparae. The effect of maternal age on very preterm birth of twins differs according to parity. To some extent, that effect is further modified by education. Therefore, future analyses of maternal age and twin birth outcomes should account for measures of obstetric history and other factors, which may influence these results.     

鄂州市某妇保医院2010-2014年早产儿发病率及其影响因素分析

目的：分析鄂州市早产的流行状况及其影响因素,为预防早产的发生提供科学依据。方法研究对象为2010年1月—2014年12月在某妇幼保健院分娩的孕满28周的孕产妇24145例,收集分娩记录资料,采用χ2检验和 Logistic 回归分析早产发生的危险因素。结果早产平均发生率为4.1%,产次(OR：1.15,95%CI ：1.01~1.30)、有流产史(OR：1.14,95%CI ：1.00~1.32)、多胎妊娠(OR：18.33,95%CI ：15.03~22.35)、男性婴儿(OR：1.19,95%CI ：1.04~1.37)是早产发生的危险因素。结论鄂州市早产率近5年来逐年下降,对于早产高危人群应加强孕期监护,预防早产的发生。     

Large social disparities in spontaneous preterm birth rates in transitional Russia

OBJECTIVE: This study estimated the effect of maternal sociodemographic, obstetric and lifestyle factors on the risk of spontaneous preterm birth in a Russian town. METHODS: All women with singleton pregnancies registered at prenatal care centres in Severodvinsk in 1999 comprised the cohort for this study (n=1559). Analysis was based on spontaneous live singleton births at the maternity home (n=1103). Multivariable logistic regression was applied to quantify the effect of the studied factors on the risk of preterm birth. Differences in gestation duration were studied using multiple linear regression. RESULTS: In total, 5.6% of all spontaneous births were preterm. Increased risks of preterm delivery were found in women with lower levels of education and in students. Placental complications, stress and a history of fetal death in previous pregnancies were also associated with elevated risks for preterm delivery. Smoking, hypertension and multigravidity were associated with reduced length of pregnancy in metric form. CONCLUSION: In addition to medical risk factors, social factors are important determinants of preterm birth in transitional Russia. Large disparities in preterm birth rates may reflect the level of inequalities in transitional Russia. Social variations in pregnancy outcomes should be monitored.     

A cohort study of traffic-related air pollution impacts on birth outcomes

BACKGROUND: Evidence suggests that air pollution exposure adversely affects pregnancy outcomes. Few studies have examined individual-level intraurban exposure contrasts. OBJECTIVES: We evaluated the impacts of air pollution on small for gestational age (SGA) birth weight, low full-term birth weight (LBW), and preterm birth using spatiotemporal exposure metrics. METHODS: With linked administrative data, we identified 70,249 singleton births (1999-2002) with complete covariate data (sex, ethnicity, parity, birth month and year, income, education) and maternal residential history in Vancouver, British Columbia, Canada. We estimated residential exposures by month of pregnancy using nearest and inverse-distance weighting (IDW) of study area monitors [carbon monoxide, nitrogen dioxide, nitric oxide, ozone, sulfur dioxide, and particulate matter < 2.5 (PM2.5) or < 10 (PM10) microm in aerodynamic diameter], temporally adjusted land use regression (LUR) models (NO, NO2, PM2.5, black carbon), and proximity to major roads. Using logistic regression, we estimated the risk of mean (entire pregnancy, first and last month of pregnancy, first and last 3 months) air pollution concentrations on SGA (< 10th percentile), term LBW (< 2,500 g), and preterm birth. RESULTS: Residence within 50 m of highways was associated with a 26% increase in SGA [95% confidence interval (CI), 1.07-1.49] and an 11% (95% CI, 1.01-1.23) increase in LBW. Exposure to all air pollutants except O3 was associated with SGA, with similar odds ratios (ORs) for LUR and monitoring estimates (e.g., LUR: OR = 1.02; 95% CI, 1.00-1.04; IDW: OR = 1.05; 95% CI, 1.03-1.08 per 10-microg/m3 increase in NO). For preterm births, associations were observed with PM2.5 for births < 37 weeks gestation (and for other pollutants at < 30 weeks). No consistent patterns suggested exposure windows of greater relevance. CONCLUSION: Associations between traffic-related air pollution and birth outcomes were observed in a population-based cohort with relatively low ambient air pollution exposure.     

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

OBJECTIVES: 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). METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.     

Socio-economic factors associated with preterm delivery. Results of the European project in Spain

Objectives: 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).Methods: 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.Results: 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.Conclusion.: 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.     

Cost effects of preterm birth: a comparison of health care costs associated with early preterm,late preterm,and full-term birth in the first 3 years after birth

Preterm birth is one of the main causes for infant morbidity and mortality. Apart from negative health outcomes, preterm birth also produces significant health care expenditures. This study evaluates the costs associated with preterm birth in different health sectors during the first 3 years of infants’ lives. In a retrospective observational study based on claims data from a German statutory health insurance company, average costs for medication, hospital treatment, ambulatory treatment, and non-medical remedies during the first 3 years after birth were analyzed for early preterm, late preterm, and full-term births. Costs associated with preterm births were generally higher than for full-term births, with the highest costs for the hospital treatment of early preterm births. Cost differences tended to decrease in the second and third year after birth except for ambulatory treatment costs, which decreased for late preterm and full-term births but not for early preterm births. The study shows that preterm birth is associated with increased health care costs, particularly during the first year after birth, indicating that the implementation of adequate programs and policies for preventing preterm birth is not only desirable from a medical but also from a health economic perspective.     

Modelling prior reproductive history to improve prediction of risk for very preterm birth

Watson LF, Rayner J‐A, King J, Jolley D, Forster D, Lumley J. Modelling prior reproductive history to improve prediction of risk for very preterm birth. Paediatric and Perinatal Epidemiology 2010. In published studies of preterm birth, analyses have usually been centred on individual reproductive events and do not account for the joint distributions of these events. In particular, spontaneous and induced abortions have often been studied separately and have been variously reported as having no increased risk, increased risk or different risks for subsequent preterm birth. In order to address this inconsistency, we categorised women into mutually exclusive groups according to their reproductive history, and explored the range of risks associated with different reproductive histories and assessed similarities of risks between different pregnancy histories. The data were from a population‐based case–control study, conducted in Victoria, Australia. The study recruited women giving birth between April 2002 and April 2004 from 73 maternity hospitals. Detailed reproductive histories were collected by interview a few weeks after the birth. The cases were 603 women who had had a singleton birth between 20 and less than 32 weeks gestation (very preterm births including terminations of pregnancy) and the controls were 796 randomly selected women from the population who had had a singleton birth of at least 37 completed weeks gestation. All birth outcomes were included. Unconditional logistic regression was used to assess the association of very preterm birth with type and number of prior abortions, prior preterm births and sociodemographic factors. Using the complex combinations of prior pregnancy experiences of women (including nulligravidity), we showed that a history of prior childbirth (at term) with no preterm births gave the lowest risk of very preterm birth. With this group as the reference category, odds ratios of more than two were associated with all other prior reproductive histories. There was no evidence of difference in risk between types of abortion (i.e. spontaneous or induced) although the risk increased if a prior preterm birth had also occurred. There was an increasing risk of very preterm birth associated with increasing numbers of abortions. This method of data analysis reveals consistent and similar risks for very preterm birth following spontaneous or induced abortions. The findings point to the need to explore commonalities rather than differences in regard to the impact of abortion on subsequent births.     

中国3省市21家医院早产发生的相关因素及结局研究

目的 分析早产发生的影响因素与结局,为早产的防治工作提供依据. 方法 对3省市21家医院完成分娩的全部产妇13 322例进行调查,计算早产发生率,分析早产发生的相关因素,比较早产儿与足月儿的新生儿结局. 结果 早产发生率为6.0％,其中晚期早产（孕周34～36周）占77.3％;导致早产发生危险增加的因素有年龄＜20岁、年龄＞35岁、在校读书年数＞12年、多胎、发生妊娠合并症;早产导致死胎或死产,低、极低出生体重,Apgar得分≤7分,进入新生儿重症监护室,出院前或生后7d内死亡的危险增加. 结论 早产是威胁胎儿及新生儿健康的重要因素,应尽早识别具备早产危险因素的孕妇,以便及时开展针对性的治疗工作.     

Association of parental characteristics with adverse outcomes of adolescent pregnancy

BACKGROUND: It is well-known that pregnancy in adolescence has an increased risk of adverse reproductive outcomes. It remains unclear whether this association is due mainly to the unfavourable sociodemographic status or due solely to biological immaturity of pregnant adolescents. OBJECTIVE: The purpose of this study was to determine the association of parental sociodemographic characteristics with the adverse outcomes of adolescent pregnancy. METHOD: Data from certificates of live births in Taichung County, Taiwan in 1994 of 7994 singleton, first-born babies whose mothers were 15-34 years of age were analysed. The relative risk of having adverse pregnancy outcomes for adolescent subgroups was obtained as compared with that among mothers 20-34 years of age with the same characteristics. The adjusted relative risk of having adverse pregnancy outcomes for each covariate was calculated by a multiple logistic regression analysis. RESULTS: Of 7994 babies born to mothers of 15-34 years of age, 8.3% were born to adolescent mothers. In all age groups, the younger adolescent mothers (15-17 years of age) had the highest percentage of both infants with low birth weight (10.6%) and preterm births (7.1 %). Younger adolescent mothers in almost all sociodemographic categories had higher risks of having both low-birth-weight and preterm births than those of older adolescent mothers. Multiple logistic regression analysis showed that a younger maternal age is the only significant risk factor for having infants with low birth weight (adjusted RR = 2.5, 95% Cl 1.8-4.5 and adjusted RR = 1.7, 95% Cl 1.2-2.6 for younger and older adolescent mothers, respectively) or preterm birth (adjusted RR = 1.9, 95% Cl 1.1- 3.4 and adjusted RR = 1.5, 95% Cl 1.0-2.3 for younger and older adolescent mothers, respectively). CONCLUSIONS: Adolescent pregnancy carries an increased risk of having low-birth-weight and preterm births, and a younger maternal age is causally implicated.      

Evaluation of antenatal risk scoring in a preterm birth prevention and perinatal loss

The study aim was to determine risk factors associated with preterm delivery, perinatal mortality, and neonatal morbidity among 687 indigent, pregnant women in their first term registered with the New Civil Hospital, Ahmedabad, India, between September, 1989, and March, 1991. Women were scored according to their level of risk: no risk, mild risk, moderate risk, and severe risk, from scores based on sociodemographic and obstetric data: pallor, maternal weight, 2 or more prior abortions, first pregnancy or 5 pregnancies, adolescent pregnancy, prior preterm birth, prior prenatal mortality or stillbirths. Out of 696 deliveries, there were 71 (10.2%) preterm births, of which 3 (2.38%) were among women within the no risk groups. There were 47 (11.10%) from the mild risk group and 20 (14.08%) from the moderate risk group. There were 20% from the severe risk group. Perinatal mortality was 84.77/1000 births, and 7.94 among the no risk group. The perinatal mortality rate rose with level of risk, with 92.20 per thousand births for the mild risks to 200 for the severe risks, which was statistically significant. Neonatal morbidity also increased with the increased level of risk. Preterm birth was found not to be associated with pallor and prior history of stillbirth. Perinatal mortality was not associated with pallor and first pregnancy. Factors significantly associated with preterm births and perinatal mortality were maternal malnutrition, higher pregnancy order, older maternal age at delivery, and prior preterm births and fetal loss. Pregnant women with risk factors had greater relative risk of preterm birth and perinatal mortality by 5.01 and 13.09 times. With maternal risk factors, the risk increased by 80.05% and 92.35%. The risk factors were highly sensitive for preterm births (95.77%), but had low specificity (19.69%), and low positive predictive value (11.93%). Perinatal mortality sensitivity, specificity, and positive predictive values were 98.31%, 19.90%, and 10.34%n respectively. The findings differed from previously reported studies; scoring system used has a higher sensitivity to predicting preterm birth and perinatal mortality among high risk women, and poor sensitivity among low risk women. Moderate and mild could be identified with this system and referred for follow-up.     

Trends in preterm births in Flanders, Belgium, from 1991 to 2002

Changes in the preterm birth rate have been attributed predominantly to increases in multiple pregnancies, associated with advanced maternal age and assisted reproduction, and to obstetric intervention. We examined their contribution to the frequencies of preterm (<37 weeks), very preterm (<32 weeks) and severely preterm (<28 weeks) birth among 700 383 singleton and twin births in Flanders from 1991 to 2002. We examined changes across four 3‐year periods (triennia) with confidence interval [CI] analysis and yearly incremental rates using linear and logistic regression analyses. Over the 12 years, twin pregnancies increased from 1.5% to 2.0%, averaging 1.6% [95% CI 1.54, 1.66] in 1991–93 and 1.9% [95% CI 1.81, 1.94] in 2000–02 (P < 0.001). The proportion of women aged 35 years or more increased from 6.8% [95% CI 6.69, 6.92] in 1991–93 to 11.3% [95% CI 11.2, 11.5] in 2000–02 (P < 0.001) and those aged under 20 from 1.9% [95% CI 1.81, 1.93] to 2.3% [95% CI 2.26, 2.41] (P < 0.001). Assisted reproduction increased from 2.6% [95% CI 2.48, 2.62] to 4.2% [95% CI 4.11, 4.30] (P < 0.001) and obstetric intervention to end pregnancy from 36.2% [95% CI 36.0, 36.4] to 40.3% [95% CI 40.1, 40.6] (P < 0.001). These increases related to an annual increase of 0.23% in the preterm birth rate from 5.5% [95% CI 5.4, 5.6] in 1991–93 to 7.2% [95% CI 7.1, 7.3] in 2000–02 (P < 0.001). The proportions of very and severely preterm births also increased by nearly a third, but their contribution to the total preterm birth rate remained stable at 15% and 5%, respectively. Odds ratios for the increases per year were 1.035 [95% CI 1.032, 1.038] for preterm birth, 1.024 [95% CI 1.018, 1.031] for very preterm and 1.028 [95% CI 1.017, 1.040] for severely preterm births after adjusting for other changes in the population. Overall, the data show, first, marked increases in the frequency of known contributors to the preterm birth rate, including twin pregnancies, advanced maternal age, assisted reproduction and obstetric intervention. Second, the preterm birth rate further increased significantly within subgroups of women with one or more of these characteristics. Third, the preterm birth rate also rose, from 4.4% [95% CI 4.2, 4.5] in 1991–93 to 5.6% [95% CI 5.5, 5.8] in 2000–02 (P < 0.001), in women with none of these contributing factors. This indicates that changes in the frequency of these known predictors are insufficient to explain the steady increase in preterm, very preterm and severely preterm births over more than a decade.     

Modelling sequence of prior pregnancies on subsequent risk of very preterm birth

Watson LF, Rayner J‐A, King J, Jolley D, Forster D, Lumley J. Modelling sequence of prior pregnancies on subsequent risk of very preterm birth. Paediatric and Perinatal Epidemiology 2010. The prevalence and intractability of preterm birth is known as is its association with reproductive history, but the relationship with sequence of pregnancies is not well studied. The data were from a population‐based case–control study, conducted in Victoria, Australia. The study recruited women giving birth between April 2002 and April 2004 from 73 maternity hospitals. Detailed reproductive histories were collected by interview a few weeks after the birth. The cases were 603 women having a singleton birth between 20 and <32 weeks gestation (very preterm births including terminations of pregnancy). The controls were 796 randomly selected women from the population having a singleton birth of at least 37 completed weeks gestation. Unconditional logistic regression was used to assess the association of very preterm birth with sequence of pregnancies defined by their outcome (prior abortion – spontaneous or induced, and prior preterm or term birth) with adjustment for sociodemographic factors. The outcomes of each prior pregnancy, stratified by pregnancy order, and starting with the pregnancy immediately before the index or control pregnancy, were categorised as one of abortion, preterm birth or term birth. We showed that each of these prior pregnancy events was an independent risk of very preterm birth. This finding does not support the hypothesis of a neutralising effect of a term birth after an abortion on the subsequent risk for very preterm birth and is further evidence for the cumulative or increasing risk associated with increasing numbers of prior abortions or preterm births.     

Rising preterm birth rates, 1989–2004: Changing demographics or changing obstetric practice?

Preterm birth rates are higher in the United States than in most industrialized countries, and have been rising steadily. Some attribute these trends to changing demographics, with more older mothers, more infertility, and more multiple births. Others suggest that changes in obstetrics are behind the trends. We sought to determine what the preterm birth rate in 2004 would have been if demographic factors had not changed since 1989. We examined complete US birth certificate files from 1989 and 2004 and used logistic regression models to estimate what the 2004 preterm birth rates (overall, spontaneous, and medically induced) would have been if maternal age, race, nativity, gravidity, marital status, and education among childbearing women had not changed since 1989. While the overall preterm births increased from 11.2% to 12.8% from 1989 to 2004, medically induced rates increased 94%, from 3.4% to 6.6%, and spontaneous rates declined by 21%, from 7.8% to 6.2%. Had demographic factors in 2004 been what they were in 1989, the 2004 rates would have been almost identical. Changes in multiple births accounted for only 16% of the increase in medically induced rates. Our analysis suggests that the increase in preterm births is more likely due primarily to changes in obstetric practice, rather than to changes in the demographics of childbearing. Further research should examine the degree to which these changes in obstetric practice affect infant morbidity and mortality.     

The Association Between Short Interpregnancy Interval and Preterm Birth in Louisiana: A Comparison of Methods

There is growing interest in the application of propensity scores (PS) in epidemiologic studies, especially within the field of reproductive epidemiology. This retrospective cohort study assesses the impact of a short interpregnancy interval (IPI) on preterm birth and compares the results of the conventional logistic regression analysis with analyses utilizing a PS. The study included 96,378 singleton infants from Louisiana birth certificate data (1995–2007). Five regression models designed for methods comparison are presented. Ten percent (10.17 %) of all births were preterm; 26.83 % of births were from a short IPI. The PS-adjusted model produced a more conservative estimate of the exposure variable compared to the conventional logistic regression method (β-coefficient: 0.21 vs. 0.43), as well as a smaller standard error (0.024 vs. 0.028), odds ratio and 95 % confidence intervals [1.15 (1.09, 1.20) vs. 1.23 (1.17, 1.30)]. The inclusion of more covariate and interaction terms in the PS did not change the estimates of the exposure variable. This analysis indicates that PS-adjusted regression may be appropriate for validation of conventional methods in a large dataset with a fairly common outcome. PS’s may be beneficial in producing more precise estimates, especially for models with many confounders and effect modifiers and where conventional adjustment with logistic regression is unsatisfactory. Short intervals between pregnancies are associated with preterm birth in this population, according to either technique. Birth spacing is an issue that women have some control over. Educational interventions, including birth control, should be applied during prenatal visits and following delivery.