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The Estimation of Gestational Age at Birth in Database Studies

Eberg, Mariaa; Platt, Robert W.b,c; Filion, Kristian B.a,b,d

doi: 10.1097/EDE.0000000000000713

Background: Studies on the safety of prenatal medication use require valid estimation of the pregnancy duration. However, gestational age is often incompletely recorded in administrative and clinical databases. Our objective was to compare different approaches to estimating the pregnancy duration.

Methods: Using data from the Clinical Practice Research Datalink and Hospital Episode Statistics, we examined the following four approaches to estimating missing gestational age: (1) generalized estimating equations for longitudinal data; (2) multiple imputation; (3) estimation based on fetal birth weight and sex; and (4) conventional approaches that assigned a fixed value (39 weeks for all or 39 weeks for full term and 35 weeks for preterm). The gestational age recorded in Hospital Episode Statistics was considered the gold standard. We conducted a simulation study comparing the described approaches in terms of estimated bias and mean square error.

Results: A total of 25,929 infants from 22,774 mothers were included in our “gold standard” cohort. The smallest average absolute bias was observed for the generalized estimating equation that included birth weight, while the largest absolute bias occurred when assigning 39-week gestation to all those with missing values. The smallest mean square errors were detected with generalized estimating equations while multiple imputation had the highest mean square errors.

Conclusions: The use of generalized estimating equations resulted in the most accurate estimation of missing gestational age when birth weight information was available. In the absence of birth weight, assignment of fixed gestational age based on term/preterm status may be the optimal approach.

Supplemental Digital Content is available in the text.

From the aCenter for Clinical Epidemiology, Lady Davis Institute, Jewish General Hospital, Montreal, QC, Canada; bDepartment of Epidemiology, Biostatistics, and Occupational Health, McGill University, Montreal, QC, Canada; cDepartment of Pediatrics, McGill University, Montreal, QC, Canada; and dDepartment of Medicine, McGill University, Montreal, QC, Canada.

Submitted January 29, 2016; accepted July 04, 2017.

Supported by an operating grant from the Canadian Institutes of Health Research (CIHR; grant number MOP 126171).

R.W.P. holds a Chercheur-National (National Scholar) Award from the Fonds de Recherche en Santé du Québec (Quebec Foundation for Health Research), and is the Albert Boehringer I Chair in Pharmacoepidemiology at McGill University. He reports personal fees from Pfizer, Novartis, Amgen, and AbbVie unrelated to this work. K.B.F. holds a New Investigator award from the CIHR. The other author has no conflicts to report.

Data Sharing: The authors are unable to provide access to the data used in this study due to the data sharing agreement with the Clinical Practice Research Datalink.

Supplemental digital content is available through direct URL citations in the HTML and PDF versions of this article (

Correspondence: Kristian B. Filion, Departments of Medicine and of Epidemiology, Biostatistics, and Occupational Health. Jewish General Hospital/McGill University. 3755 Cote Ste-Catherine Road, Suite H416.1. Montreal, Quebec, Canada. E-mail:

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