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  • Original Article
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Standard methods based on last menstrual period dates misclassify and overestimate US preterm births




To compare number of US preterm births based on obstetric versus last menstrual period (LMP) estimates and evaluate their correlations with clinical risk indicators associated with prematurity

Study Design:

Preterm births were assessed from LMP, per standard practice, and, separately, from obstetric estimates using the 2012 Natality Public Use File. Percentages of infants with neonatal intensive care unit (NICU) admission and low birth weight (LBW) were calculated.


More births were <37 weeks gestational age (GA) by reported LMP (11.4%) versus obstetric estimates (9.5%). Among infants preterm by LMP, but born at 37–41 weeks by obstetric estimates, there were 5.7% NICU admission and 7.7% LBW rates versus 25.2% and 35.4%, respectively, of those preterm by obstetric estimates but born 37–41 weeks by LMP assessments.


Obstetric estimates provide the most clinically relevant estimates of US preterm births. Assessments calculated from LMP alone may overestimate prematurity incidence by ~20%.

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Medical writing assistance was provided by John E. Fincke and Anny Wu of Complete Healthcare Communications, Inc., (Chadds Ford, PA, USA) and funded by MedImmune. This study was sponsored by MedImmune.

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Correspondence to C Rizzo.

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CSA, HC, CR and GK are employees of AstraZeneca and may have stock or stock options. ECS has no conflict of interest to declare.

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Supplementary Information accompanies the paper on the Journal of Perinatology website

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Ambrose, C., Caspard, H., Rizzo, C. et al. Standard methods based on last menstrual period dates misclassify and overestimate US preterm births. J Perinatol 35, 411–414 (2015).

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