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