Trends in preterm birth and neonatal mortality among blacks and whites in the United States from 1989 to 1997

Preterm birth, a major determinant of infant mortality, has been increasing in recent years. The authors examined trends in preterm birth and its determinants by using the US birth and infant death files for 1989-1997. The impact of trends in preterm birth rates on neonatal and infant mortality was also evaluated. Among Whites, preterm births (<37 completed weeks of gestation) increased from 8.8% of livebirths in 1989 to 10.2% in 1997, a relative increase of 15.6%. On the other hand, preterm births among Blacks decreased by 7.6% (from 19.0% to 17.5%) during the same period. An increase in obstetric interventions contributed to increases in preterm births for both races but was outweighed by other unidentified favorable influences for Blacks. Neonatal mortality among preterm Whites dropped 34% during the 8 years of the study, while the decrease was only 24% among Blacks. This large disparity countered the changes in preterm birth rates so that the percentage decline in neonatal mortality was similar in the two racial groups (18-20%). In conclusion, the anticipated mortality benefit from a lower preterm birth rate for Blacks has been blunted by suboptimal improvement in mortality among the remaining preterm infants. The widening race gap in mortality among preterm infants merits attention.     

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.     

Changing patterns of low birthweight and preterm birth in the United States, 1981-98

Low birthweight (LBW) and preterm birth are primary risk factors for infant morbidity and mortality in the US. With increasing multiple births and delayed childbearing, it is important to examine the separate contributions of these characteristics to the increases in LBW and preterm birth rates. US natality records from 1981, 1990 and 1998 were used to calculate LBW (% births <1500, 1500-2499, <2500 g) and preterm (% births <29, 29-32, 33-36, <37 weeks gestation) rates. Data were stratified by maternal race (black or white) and plurality (singleton vs. multiple birth). LBW and preterm rates among singletons were adjusted for maternal age to examine the influence of demographic shifts on LBW trends. From 1981 to 1998, LBW increased 12% among white infants, but remained relatively stable among black infants. During the same time, preterm birth increased 23% among white infants compared with 3% among black infants. For both black and white infants, the increase in LBW and preterm births was greater among multiple births than among singletons. Adjustment for maternal age did not reduce the temporal increase in LBW or preterm birth among singletons. Black infants continue to experience a markedly higher incidence of LBW and preterm birth, but the racial gap in these outcomes has narrowed slightly in recent years as a result of increasing LBW and preterm birth among white births. The differing trends for white and black infants are the consequence of a disparate trend in the incidence and outcome of multiple births coupled with increases in LBW and preterm birth among white singletons. Understanding the differential patterns in birth outcomes among white and black infants is necessary to develop effective interventions designed to decrease racial disparities in pregnancy outcome.     

Increasing social variation in birth outcomes in the Czech Republic after 1989.

OBJECTIVES: This study investigated social variation in birth outcome in the Czech Republic after the political changes of 1989. METHODS: Routinely collected records on singleton live births in 1989, 1990, and 1991 (n = 380,633) and 1994, 1995, and 1996 (n = 286,907) were individually linked to death records. RESULTS: Mean birthweight fell from 3,323 g to 3,292 g (P < .001) between 1989 and 1991 and then increased to 3,353 g by 1996. The gap in mean birthweight between mothers with a primary education and those with a university education, adjusted for age, parity, and sex of infants, widened from 182 g (95% confidence interval [CI] = 169, 19) in 1989 to 256 g (95% CI = 240, 272) in 1996. Similar trends were found for preterm births. Postneonatal mortality declined most among the better educated and the married. The odds ratio for postneonatal death for infants of mothers with a primary (vs university) education, adjusted for birthweight, increased from 1.99 (95% CI = 1.52, 2.60) in 1989 through 1991 to 2.39 (95% CI = 1.55, 3.70) in 1994 through 1995. CONCLUSIONS: Despite general improvement in the indices of fetal growth and infant survival in the most recent years, social variation in birth outcome in the Czech Republic has increased.     

Social and racial inequalities in preterm births in Western Australia, 1984 to 2006

Langridge AT, Nassar N, Li J, Stanley FJ. Social and racial inequalities in preterm births in Western Australia, 1984 to 2006. Paediatric and Perinatal Epidemiology 2010. Preterm birth is associated with a range of childhood morbidities and in industrialised societies is the primary cause of infant mortality. Social and racial inequalities in preterm birth have been reported in North America, UK, Europe and New Zealand. This study utilised population‐level data to investigate social and racial inequalities in preterm birth among Aboriginal and non‐Aboriginal infants in Western Australia. All live, singleton births between 1984 and 2006 (n = 567 468) were included, and multilevel multivariable logistic regression was used to investigate relative differences in preterm infants between socio‐economic groups. Aboriginal and non‐Aboriginal infants were analysed separately. The prevalence of preterm births increased from 7.1% in 1984–88 to 7.5% in 1999–2003, before decreasing to 7.2% in 2004–06. Inequalities in preterm births between Aboriginal and non‐Aboriginal infants increased over time, with the percentage of preterm births being almost twofold higher for Aboriginal infants (14.8%), compared with non‐Aboriginal infants (7.6%). A significant portion of the disparity between Aboriginal and non‐Aboriginal infants is attributable to parental socio‐economic and demographic characteristics, though the disparity continues to persist even after adjustment for these factors. While the overall rates of preterm birth in Western Australia have remained fairly static over the last two decades, the disparity between Aboriginal and non‐Aboriginal infants has increased and is now similar to inequalities seen 20 years ago. These findings highlight a major public health issue that should be of great concern, given the short‐ and long‐term morbidities and complications associated with preterm birth.     

Preterm delivery rates in North Carolina: are they really declining among non-Hispanic African Americans?

The preterm delivery rate in North Carolina is consistently higher than the national average. However, recent reports suggest that singleton preterm delivery rates for non-Hispanic Whites are increasing while those for non-Hispanic African Americans are decreasing. To study this pattern further, the authors examined data on singleton non-Hispanic White and non-Hispanic African-American births in 1989 and 1999 by using North Carolina vital statistics data. They found that the frequency of preterm delivery rose 1.1% (8.5% to 9.6%) among non-Hispanic Whites but declined 1.4% (17.9% to 16.5%) among non-Hispanic African Americans over the same time period. For both subgroups, a bimodal distribution of birth weights was apparent among preterm births at 28-31 weeks of gestation. The second peak with its cluster of normal-weight infants was more prominent among non-Hispanic African Americans in 1989 than in 1999. To reduce the potential for bias due to misclassification of infant gestational age, frequencies of preterm delivery of infants who weighed less than 2,500 g were calculated. Unlike the original analysis, this calculation showed that preterm delivery increased for both subgroups. A number of non-Hispanic African-American births classified as preterm were apparently term births mistakenly assigned short gestational ages. Such misclassification was more frequent in 1989 than in 1999, inflating 1989 preterm delivery rates.     

Impact of multiple births on low birthweight--Massachusetts, 1989-1996

In Massachusetts during 1989-1996, perinatal health indicators such as infant mortality, teen birth rate, and maternal smoking during pregnancy decreased steadily; however, low birthweight (LBW) (i.e., <2500 g [<5 lbs, 8 oz]) rates increased consistently. During this same period, the multiple-birth rate (i.e., number of twins and higher order multiple births per 100 live births) increased from 2.5% in 1989 to 3.5% in 1996. Massachusetts has the highest multiple-birth rate in the United States. Multiple births are more likely to result in LBW infants. To determine the effect of changes in the rate of multiple births on LBW rates and to characterize women who have multiple births, the Massachusetts Department of Public Health examined data on births in Massachusetts during 1989-1996. This report summarizes the results of this analysis, which indicate that the increase in LBW rates in Massachusetts was associated with changes in the rate of multiple births, especially among older, better educated women.     

中国10县(市)1993-2005年单胎儿早产流行状况研究

目的描述中国10县（市）单胎儿早产流行状况及长期变化趋势。方法资料来源于“中美预防出生缺陷和残疾合作项目”中的围产保健监测数据库。研究对象为3省10县（市）在1993—2005年间所有孕满28周且分娩单胎儿（包括死胎死产）的孕产妇542923名，计算早产率并描述其分布特征。结果1993—2005年间在10县（市）共发生早产25784例（其中死胎死产1530例），早产率为4，75％（95％CI：4.69～4.81），活产儿早产率为4．49％（95％CI：4．44～4．55）。单胎早产率呈逐年下降趋势，无明显季节倾向；南方城市、南方农村和北方农村的早产率依次降低；分娩年龄与早产率呈“U”型关系；文化程度低、孕产次多、有早产史或自然流产史者早产率较高。结论10县（市）单胎早产率呈逐年下降趋势，不同地区差别显著。     

Secular trends of fetal growth in Canada, 1981 to 1997

Preterm birth and low birthweight in Canada have shown paradoxical temporal trends, with an increase in preterm birth and a decrease in low birthweight. Mean birthweight has increased in many industrialised countries, despite a recent rise in preterm birth, suggesting a temporal increase in fetal growth (birthweight for gestational age) in Canada. We thus described temporal trends in the distribution of fetal growth from 1981 to 1997, including means and proportions of infants at both the low and high ends of the fetal growth distribution. We used data for singleton live births from Statistics Canada's Canadian Birth Data Base for the years 1981-97 (excluding Ontario and Newfoundland) and analysed temporal trends in birthweight and birthweight-for-gestational-age z-score as continuous outcomes and the derived dichotomised outcomes [i.e. low birthweight (<2500 g), very low birthweight (<1500 g), small-for-gestational-age (<10th percentile), very small-for-gestational-age (<3rd percentile), high birthweight (>4000 g), very high birthweight (>4500 g), large-for-gestational-age (>90th percentile), and very large-for-gestational-age (>97th percentile)]. The birthweight-for-gestational-age was based on a newly developed population-based Canadian reference. The results showed that in the overall sample and in a subsample of term and post-term births, mean birthweight, mean z-score, rates of high birthweight, very high birthweight, large-for-gestational-age, and very large-for-gestational-age increased whereas rates of low birthweight, very low birthweight, small-for-gestational-age, and very small-for-gestational-age decreased between 1981-83 and 1995-97. The reverse was observed in preterm births. These temporal changes were larger for more extremely distributed measures of fetal growth. For example, compared with 1981-83, the decrease in 1995-97 for very small-for-gestational-age (<3rd percentile) was 38.9%, whereas the decrease for small-for-gestational-age (<10th percentile) was only 29.7%. Corresponding temporal increases were 21.4% for very large-for-gestational-age (>97th percentile) and 15.2% for large-for-gestational-age (>90th percentile). Among infants with gestational age 34-36 weeks, all measures of fetal growth, including the rates for all dichotomous outcomes, decreased in 1995-97 as compared with 1981-83. We conclude that Canadian infants are getting bigger, but only those born at term. The temporal trends for more extremely distributed fetal growth measures are particularly marked.     

Contribution of assisted reproductive technology and ovulation-inducing drugs to triplet and higher-order multiple births--United States, 1980-1997

In the United States, pregnancies associated with assisted reproductive technology (ART) or ovulation-inducing drugs are more likely to result in multiple births than spontaneously conceived pregnancies (1). In addition, triplet and higher-order multiple births are at greater risk than singleton births to be preterm (< or = 37 completed weeks' gestation), low birthweight (LBW) (i.e., < or = 2500 g), or very low birthweight (i.e., < 1500 g), resulting in higher infant morbidity and mortality (2). Because preterm and LBW infants often require costly neonatal care and long-term developmental follow-up, the continuing increase in triplet and higher-order multiple births causes concern among health-care providers and policymakers (3). This report provides estimates of the contribution of ART and ovulation-inducing drugs to these birth outcomes for 1996 and 1997, and summarizes trends during 1980-1997, which indicate that the ratio of triplet and higher-order multiple births has more than quadrupled and that a large proportion of this increase can be attributed to ART or the use of ovulation-inducing drugs.     

Obstetrical Intervention and the Singleton Preterm Birth Rate in the United States From 1991�C2006

Objectives. We examined the relationship between obstetrical intervention and preterm birth in the United States between 1991 and 2006.Methods. We assessed changes in preterm birth, cesarean delivery, labor induction, and associated risks. Logistic regression modeled the odds of preterm obstetrical intervention after risk adjustment.Results. From 1991 to 2006, the percentage of singleton preterm births increased 13%. The cesarean delivery rate for singleton preterm births increased 47%, and the rate of induced labor doubled. In 2006, 51% of singleton preterm births were spontaneous vaginal deliveries, compared with 69% in 1991. After adjustment for demographic and medical risks, the mother of a preterm infant was 88% (95% confidence interval [CI] = 1.87, 1.90) more likely to have an obstetrical intervention in 2006 than in 1991. Using new birth certificate data from 19 states, we estimated that 42% of singleton preterm infants were delivered via induction or cesarean birth without spontaneous onset of labor.Conclusions. Obstetrical interventions were related to the increase in the US preterm birth rate between 1991 and 2006. The public health community can play a central role in reducing medically unnecessary interventions.During the past 15 years, rates of obstetrical interventions have been rising in the United States.^1,2 The percentage of births with induced labor more than doubled between 1991 and 2006, from 10.5% to 22.5%.^1,2 After a decline in the early 1990s, the cesarean delivery rate increased by 50%, from 20.7% in 1996 to an all-time high of 31.1% in 2006.¹ Large increases occurred for both primary and repeat cesarean deliveries and among mothers with no known medical risk factors or indications for cesarean delivery (such as diabetes, hypertension, or premature rupture of membranes).^1,3,4 Recent studies have shown that changing primary cesarean rates did not correspond to shifts in mothers’ medical risk profiles but, rather, appeared to be related to increased use of cesarean delivery with all medical conditions.^4–6From 1991 to 2006, the preterm (less than 37 weeks of gestation) birth rate increased by 19%, from 10.8% to 12.8% of all births¹; the preterm rate increased by 13% for singletons and by 22% for multiple births. An increase in the preterm birth rate is of concern because rates of death and disability are higher among preterm infants than among infants born at term (37–41 weeks).^7–9 Although rates of death and disability are highest among infants born very preterm (less than 32 weeks), mortality rates among moderately preterm (32–33 weeks) and late preterm (34–36 weeks) infants are 7 and 3 times, respectively, the mortality rates for term infants.⁷We examined the relationship between changes in the use of obstetrical intervention and changes in the preterm birth rate in the United States between 1991–2006. Specifically, we explored trends in singleton preterm births, delivery methods (cesarean or vaginal), and induction of labor.     

Is paternal age associated with an increased risk of low birthweight,preterm delivery,and multiple birth?

OBJECTIVE: To determine if paternal age elevates the risk of low birthweight (< 2500g, LBW), preterm birth (< 37 weeks gestation), and multiple gestation among mothers whose age does not predict an elevated risk. DESIGN/METHODS: Population data on birth outcome, maternal age and paternal age was obtained from Alberta Health and Wellness for all births 1990-1996. RESULTS: Among women aged 25 to 29, regardless of parity, there was no linear relationship between paternal age and preterm birth or LBW. However, multiple birth rates increased with increased paternal age (p < 0.01). Among singleton births, advanced paternal age (>50 years) increased the risk of LBW and preterm birth (p < 0.05). CONCLUSIONS: Paternal age is not a risk factor for LBW or preterm delivery among low risk women. The increased risk of multiple birth with increased paternal age, regardless of parity, requires confirmation among other populations.     

Preterm Birth Among American Indian/Alaskan Natives in Washington and Montana: Comparison with Non-Hispanic Whites

Preterm birth is the single most important cause of perinatal mortality in North America. Given that American Indians/Alaskan Natives (AI/ANs) in the United States continue to have adverse birth outcomes, the purpose of this study is to compare the risk of preterm birth among AI/AN mothers to Non-Hispanic White mothers living in Washington and Montana from 2003 to 2009. A population-based retrospective cohort study was conducted examining the association between AI/AN mothers (self-reported) and the risk of preterm birth (gestational age <37 weeks) using birth certificate data from Washington and Montana. All AI/AN singleton lives births (n = 26,648) from residents of Washington and Montana from 2003 to 2009 were identified and included in our study. An identical number of Non-Hispanic White singleton infants (n = 26,648) born to residents of Washington and Montana were randomly selected as a comparison group and logistic regression was used to analyze the data. AI/AN mothers living in Washington and Montana between 2003 and 2009 were 1.34 times (95 % CI 1.25–1.44) as likely to have a preterm birth compared to Non-Hispanic Whites after adjusting for maternal and paternal characteristics as well as pregnancy risk factors. AI/AN mothers residing in Washington and Montana from 2003 to 2009 were at a significantly increased risk of having a preterm birth compared to Non-Hispanic Whites. Identifying etiologic differences in preterm birth experienced by AI/ANs is essential in targeting future interventions.     

Infants with single umbilical artery studied in a national registry. General epidemiological characteristics

Summary. In this paper, registry data on infants born in Sweden between 1983 and 1986 are reviewed to describe the epidemiological characteristics of infants with single umbilical artery (SUA). During this period 372 066 births were registered with information on the number of umbilical vessels. Our data set contains 1782 SUA infants. The incidence at birth was: in multiple births 0.8%, in infants with chromosome anomalies 6.1% and in singletons without a known chromosome anomaly 0.46%. Incidence was higher in girls than in boys. There were no consistent seasonal variations in the date of presumed conception. Low birthweight (<2500 g) and preterm birth (<37 weeks) were seen more frequently in SUA singleton infants than in infants with three vessels. At any given gestation, SUA infants had a lower mean birthweight than infants with three vessels. The risk of having a SUA infant was increased in women over 40 years, and slightly increased at or above parity of three.     

Infants with single umbilical artery studied in a national registry. General epidemiological characteristics

In this paper, registry data on infants born in Sweden between 1983 and 1986 are reviewed to describe the epidemiological characteristics of infants with single umbilical artery (SUA). During this period 372,066 births were registered with information on the number of umbilical vessels. Our data set contains 1782 SUA infants. The incidence at birth was: in multiple births 0.8%, in infants with chromosome anomalies 6.1% and in singletons without a known chromosome anomaly 0.46%. Incidence was higher in girls than in boys. There were no consistent seasonal variations in the date of presumed conception. Low birthweight (less than 2500 g) and preterm birth (less than 37 weeks) were seen more frequently in SUA singleton infants than in infants with three vessels. At any given gestation, SUA infants had a lower mean birthweight than infants with three vessels. The risk of having a SUA infant was increased in women over 40 years, and slightly increased at or above parity of three.     

Impact of multiple births and elective deliveries on the trends in low birth weight in Norway, 1967-1995.

To describe trends in low birth weight (less than 2,500 g), the authors analyzed 1.7 million live births and stillbirths registered between 1967 and 1995 in the Medical Birth Registry of Norway. The proportion of low birth weight infants declined from 5.3% in 1967 to 4.5% in 1979 and was followed by a steady increase that reached 5.3% in 1995. Similar trends were observed in the proportion of preterm births. Mean birth weight increased from 3,456 g in 1967 to 3,518 g in 1995. From 1979 to 1987, the increase in the prevalence of low birth weight was related to single births, and after 1987 it was related to multiple births, which increased from 2.3% of all births in 1987 to 3.1% in 1995. The proportion of low birth weight in births occurring after 37 weeks of gestation declined continuously, resulting in low birth weight births' to an increasing extent being made up of births occurring before 37 weeks of gestation. In an ecologic analysis based on county of maternal residence, the increase in low birth weight among single births was accounted for by an increase in deliveries with induction of labor or cesarean section. The authors conclude that the overall proportion of low birth weight births is not a good indicator of health in a population with extensive use of obstetric procedures that affect gestational age or assisted fertilization, which increases the number of multiple births.     

Effect of Modifiable Risk Factors on Preterm Birth: A Population Based-Cohort

Objectives The purpose of this study is to evaluate the prevalence, impact, and interaction of short interpregnancy interval (IPI), pre-pregnancy body mass index (BMI) category, and pregnancy weight gain (PWG) on the rate of preterm birth. Methods This is a population-based retrospective cohort study using vital statistics birth records from 2006 to 2011 in OH, US, analyzing singleton live births to multiparous mothers with recorded IPI (n?=?393,441). Preterm birth rate at <37 weeks gestational age was compared between the referent pregnancy (defined as normal pre-pregnancy maternal BMI, IPI of 12–24 months, and Institute of Medicine (IOM) recommended PWG) and those with short or long IPI, abnormal BMI (underweight, overweight, and obese), and high or low PWG (under or exceeding IOM recommendations). Results Only 6?% of the women in this study had a referent pregnancy, with a preterm birth rate of 7.6?% for this group. Short IPIs of <6 and 6–12 months were associated with increased rates of preterm birth rate to 12.9 and 10.4?%, respectively. Low PWG compared to IOM recommendations for pre-pregnancy BMI class was also associated with increased preterm birth rate of 13.2?% for all BMI classes combined. However, the highest rate of preterm birth of 25.2?% occurred in underweight women with short IPI and inadequate weight gain with _adjOR 3.44 (95?% CI 2.80, 4.23). The fraction of preterm births observed in this cohort that can be attributed to short IPIs is 5.9?%, long IPIs is 8.3?%, inadequate PWG is 7.5?%, and low pre-pregnancy BMI is 2.2?%. Conclusions Our analysis indicates that a significant proportion of preterm births in Ohio are associated with potentially modifiable risk factors. These data suggest public health initiatives focused on preterm birth prevention could include counseling and interventions to optimize preconception health and prenatal nutrition.     

Risk factors for the increasing trend in low birth weight among live births born by vaginal delivery, Brazil

OBJECTIVE: To identify risk factors for low birth weight (LBW) among live births by vaginal delivery and to determine if the disappearance of the association between LBW and socioeconomic factors was due to confounding by cesarean section. METHODS: Data were obtained from two population-based cohorts of singleton live births in Ribeir?o Preto, Southeastern Brazil. The first one comprised 4,698 newborns from June 1978 to May 1979 and the second included 1,399 infants born from May to August 1994. The risks for LBW were tested in a logistic model, including the interaction of the year of survey and all independent variables under analysis. RESULTS: The incidence of LBW among vaginal deliveries increased from 7.8% in 1978--79 to 10% in 1994. The risk was higher for: female or preterm infants; newborns of non-cohabiting mothers; newborns whose mothers had fewer prenatal visits or few years of education; first-born infants; and those who had smoking mothers. The interaction of the year of survey with gestational age indicated that the risk of LBW among preterm infants fell from 17.75 to 8.71 in 15 years. The mean birth weight decreased more significantly among newborns from qualified families, who also had the highest increase in preterm birth and non-cohabitation. CONCLUSIONS: LBW among vaginal deliveries increased mainly due to a rise in the proportion of preterm births and non-cohabiting mothers. The association between cesarean section and LBW tended to cover up socioeconomic differences in the likelihood of LBW. When vaginal deliveries were analyzed independently, these socioeconomic differences come up again.     

State-Specific Trends in Preterm Delivery: Are Rates Really Declining Among Non-Hispanic African Americans Across the United States?

Objectives: This study sought to examine state-specific trends in preterm delivery rates among non-Hispanic African Americans and to assess whether these rates are influenced by misclassification of gestational age. Methods: The sample population consisted of singleton non-Hispanic White and non-Hispanic African–American infants born in 1991 and 2001 to U.S. resident mothers. For both time periods, state-specific and national preterm delivery rates were calculated for all infants, stratified by infant race/ethnicity. Next, birth-weight distributions within strata of gestational age were studied to explore possible misclassifications of gestational age. Lastly, state-specific and national preterm delivery rates among infants who weighed less than 2,500 g were separately computed. Results: National analyses showed that the frequency of preterm delivery increased by 15.8% among non-Hispanic Whites but declined by 10.3% among non-Hispanic African Americans over the same period. For both subgroups, a bimodal distribution of birth weights was apparent among preterm births at 28–31 weeks of gestation. The second peak with its cluster of normal-weight infants was more prominent among non-Hispanic African Americans in 1991 than in 2001. After excluding preterm infants who weighed 2,500 g or more, the national trends persisted. State-specific analyses showed that preterm delivery rates increased for both subgroups in 13 states during this period. Of these 13, 6 states had a number of non-Hispanic African–American births classified as preterm that were apparently term births mistakenly assigned short gestational ages. Such misclassification was more frequent in 1991 than in 2001 and inflated 1991 rates. Conclusion: There is heterogeneity in state-specific preterm delivery rates. Such differences are often overlooked when aggregate results are presented.     

The prevalence of preterm birth and season of conception

Preterm birth is a major obstetric problem. An exploration of the season of conception in relation to preterm birth may provide direction in the search for risk factors. We conducted a retrospective cohort study of 82 213 singleton livebirths (20–45 weeks' gestation) to 61 630 women at Magee‐Womens Hospital, Pittsburgh, PA, from 1995 to 2005. Conception was estimated based on gestational age determined by best obstetric estimate. Fourier series analysis was used to model seasonal trends. Spontaneous preterm birth at <37 weeks was associated with conception season (P < 0.05). The peak prevalence occurred among conceptions in winter and spring (peaking February 23 at 6.9%), with an average trough among late summer/early autumn conceptions (August 25 at 6.2%). The pattern for spontaneous preterm birth <32 weeks was similar (P < 0.05), with the peak on March 13 (1.7%), and nadir on September 12 (1.4%). Results were similar when indicated preterm births were included. These seasonal changes may increase our insight into the role of exposures with seasonal periodicity in the pathophysiology of preterm birth.