Comparison of gestational dating methods and implications for exposure–outcome associations: an example with PM2.5and preterm birth
比较了基于末次月经和临床估算两种孕周方法对早产风险估计的影响，发现方法选择会导致结果差异，尤其当暴露和结局都依赖孕周时需谨慎。
<h3>Objectives</h3> 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. <h3>Methods</h3> 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 10<sup>6</sup> pregnancies) and risk differences (RD (95% CIs)) associated with exposure to particulate matter (PM<sub>2.5</sub>). <h3>Results</h3> 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 PM<sub>2.5</sub> 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. <h3>Conclusions</h3> 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.