Objectives Estimating gestational age is usually based on date of last menstrual period (LMP) or clinical estimation (CE); both approaches introduce potential bias. Differences in methods of estimation may lead to misclassification and inconsistencies in risk estimates, particularly if exposure assignment is also gestation-dependent. This paper examines a ‘what-if’ scenario in which alternative methods are used and attempts to elucidate how method choice affects observed results.
Methods We constructed two 20-week gestational age cohorts of pregnancies between 2000 and 2005 (New Jersey, Pennsylvania, Ohio, USA) using live birth certificates: one defined preterm birth (PTB) status using CE and one using LMP. Within these, we estimated risk for 4 categories of preterm birth (PTBs per 106 pregnancies) and risk differences (RD (95% CIs)) associated with exposure to particulate matter (PM2.5).
Results More births were classified preterm using LMP (16%) compared with CE (8%). RD divergences increased between cohorts as exposure period approached delivery. Among births between 28 and 31 weeks, week 7 PM2.5 exposure conveyed RDs of 44 (21 to 67) for CE and 50 (18 to 82) for LMP populations, while week 24 exposure conveyed RDs of 33 (11 to 56) and −20 (−50 to 10), respectively.
Conclusions Different results from analyses restricted to births with both CE and LMP are most likely due to differences in dating methods rather than selection issues. Results are sensitive to choice of gestational age estimation, though degree of sensitivity can vary by exposure timing. When both outcome and exposure depend on estimate of gestational age, awareness of nuances in the method used for estimation is critical.
- gestational dating
- environmental exposure
- particulate matter
- preterm birth
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Contributors KMR conceived and designed the study and statistical analysis plan with substantial input from DTL, LCM, CP and JLD. KMR acquired, cleaned, linked and analysed data, and drafted and revised the manuscript. DTL acquired data, assisted with data interpretation and revised the manuscript. LCM, CP and JLD assisted with data interpretation and revised the manuscript. JLD supervised the study design and statistical analysis plan. KMR is the guarantor.
Funding This project was supported in part by an appointment to the Internship/Research Participation Programme at the Office of Research and Development, US Environmental Protection Agency, administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the US Department of Energy and EPA.
Competing interests None declared.
Ethics approval University of North Carolina Chapel Hill, Office of Human Research Ethics.
Provenance and peer review Not commissioned; externally peer reviewed.