Objective: Preterm and postterm birth rates are substantially higher in the United States than in Canada and other industrialized countries, although relative mortality at preterm compared with term gestation is considerably lower. We attempted to explain these differences based on differences in the method of gestational age estimation.
Methods: We used information on all live births in the United States and Canada for 1995-2002 and on singleton births and perinatal deaths for 1996-1999. Gestational age in Canada was based on the clinical estimate, whereas in the United States both menstrual-based and clinical estimates were used.
Results: In 2002, preterm (12.3%) and postterm birth (6.6%) rates in the United States were far higher than in Canada (7.6% and 1.0%, respectively) when U.S. rates were based on menstrual dates. Differences were reduced or abolished when U.S. rates were based on the clinical estimate of gestation (10.1% and 1.0%, respectively). In Canada, the rate ratio for perinatal death at preterm compared with term gestation was 27.8 (95% confidence interval [CI] 26.3-29.3), similar to that in the United States when gestation was based on the clinical estimate (rate ratio 26.5, 95% CI 26.1-26.9, P value for difference in rate ratios=.06) but not when based on menstrual dates (rate ratio 18.9, 95% CI 18.7-19.2, P<.001).
Conclusion: Menstrual dates in U.S. data misclassify gestational duration and overestimate both preterm and postterm birth rates. For international comparisons, gestational age in the United States should be based on the clinical estimate.
Level of evidence: II.