Changes in the Gestational Age Distribution among U.S. Singleton Births: Impact on Rates of Late Preterm Birth, 1992 to 2002

https://doi.org/10.1053/j.semperi.2006.01.009Get rights and content

There is mounting evidence that infants born late preterm (34-36 weeks) are at greater risk for morbidity than term infants. This article examines the changing epidemiology of gestational length among singleton births in the United States, from 1992 to 2002. Analyzing gestational age by mode of delivery, the distribution of spontaneous births shifted to the left, with 39 weeks becoming the most common length of gestation in 2002, compared with 40 weeks in 1992 (P < 0.001). Deliveries at ≥40 weeks gestation markedly decreased, accompanied by an increase in those at 34 to 39 weeks (P < 0.001). Singleton births with PROM or medical interventions had similar trends. Changes in the distribution of all singleton births differed by race/ethnicity, with non-Hispanic white infants having the largest increase in late preterm births. These observations, in addition to emerging evidence of increased morbidity, suggest the need for investigation of optimal obstetric and neonatal management of these late preterm infants.

Section snippets

Materials and Methods

Analyses were based on the U.S. National Center for Health Statistics (NCHS) natality data from 1992, 1997, and 2002. U.S. natality files contain data from all certificates of live births that were submitted through the Vital Statistics Cooperative Program.2 Only singletons were selected for analysis. To minimize variability in reporting over the study period, and to reduce misclassification errors, records were excluded if the gestational age was <23 weeks, >44 weeks, or if birth weight was

Results

In 2002, there were 3,808,473 singleton live births, of which 394,996 (10.4%) were delivered preterm. The largest proportion of these preterm births were delivered at 36 completed weeks gestation (40.1%), with infants delivered between 34 and 36 completed weeks accounting for about 74% of the total number of preterm births (Fig. 1). Very preterm births comprised 13.6% of all the eligible singleton preterm births.

The proportion of infants born at 40 to 44 weeks gestation within all delivery

Discussion

Traditionally, Naegele’s rule has been used in clinical practice to estimate gestational age, stating that the duration of pregnancy for women with a regular 28-day cycle should be 280 days (40 weeks) after LMP.17 Other studies that collectively examined more than 400,000 singleton births found the median duration of human spontaneous singleton pregnancies to be 282 days following LMP.18, 19

Our analysis of singleton births documents that 39 rather than 40 weeks has become the most common length

Conclusion

In summary, there has been a considerable shift over the past decade in the distribution of births away from postterm births and toward earlier gestational ages, including substantial increases in earlier term and especially preterm birth rates, but not in very preterm births. This trend is true for all three categories of delivery examined here. The increase in births at 34 to 36 weeks is especially notable in non-Hispanic white infants. Clearly, these changes have been accompanied by

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