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Comparison of gestational dating methods and implications for exposure-outcome associations: an example with PM2.5 and preterm birth
Citation:
Rappazzo, K., D. Lobdell, L. Messer, C. Poole, AND J. Daniels. Comparison of gestational dating methods and implications for exposure-outcome associations: an example with PM2.5 and preterm birth. JOURNAL OF OCCUPATIONAL AND ENVIRONMENTAL MEDICINE. Lippincott Williams & Wilkins, Philadelphia, PA, 74(2):138-143, (2017).
Impact/Purpose:
In research examining pregnancy-based outcomes and exposures, the estimation of gestational age (GA) is a critical step. It is important to understand how the method of gestational dating used in analyses may affect estimated associations This study shows that choice of gestational dating method (clinical estimate or last menstrual period) may influence effect estimation through either outcome or exposure timing. This provides important methodological clarification that will help improve scientific interpretability of future studies of environmental effects on birth outcomes.
Description:
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 misclassificat ion 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% Cl s)) 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.
