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Influence of Gestational Age Measurement Technique on the Associations Between Maternal Characteristics and Birth
Ginna L Doss1,2, Julie L Daniels2, Sunni L Mumford3
1Epidemiology Branch, National Institute of Environmental Health Sciences (NIEHS), National Institutes of Health (NIH), Department of Health and Human Services (DHHS), North Carolina, USA.
Background:
Associations of factors of interest with preterm birth and small for gestational age (SGA) may be impacted by the technique used to measure gestational age at birth.
Objective:
To compare the associations of several factors with preterm birth and SGA when gestational age was measured with three different techniques.
Methods:
Among participants with a live birth in the Effects of Aspirin in Gestation and Reproduction (EAGeR) trial, this study used three methods to estimate gestational age: 1) prospective fertility monitor ovulation, 2) prospective LMP, and 3) early ultrasound. The study assigned the outcomes of preterm birth and SGA by each method among participants with complete gestational age information (N = 392). The study reported unadjusted prevalence differences (PDs) and 95% confidence intervals (CIs) for preterm birth and SGA, by each method, with two commonly measured maternal characteristics: age ≥ 35 years (versus < 35) and body mass index ≥ 25 kg/m2 (versus < 25), and with aspirin treatment (versus placebo).
Results:
For all but one characteristic-outcome pair examined, unadjusted PDs were similar, within 3 percentage points, for all gestational age measurement techniques. The largest discrepancy among the PDs was in the association of age ≥ 35 years with preterm birth: PD = 6% (95% CI: -6, 17) when defined by ovulation, 1% (95% CI: -10, 11) by ultrasound, and 2% (95% CI: -8, 11) by LMP. Precision of the PDs, as assessed by their confidence limit differences, did not identify a consistently more precise gestational age measurement technique.
Conclusions:
Even when gestational age was measured prospectively and early in pregnancy, small differences in the classification of preterm birth and SGA were found. These differences have the potential to bias the association of maternal characteristics with these outcomes. This should be considered by researchers, particularly in populations where the gestational age assignment technique is unknown.
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