Pregnancy Length Measurement Error: A comparison of Last Menstrual Period and Ultrasonography with Ovulation-Based Estimation

Background: Last menstrual period (LMP) and ultrasound are commonly used to estimate pregnancy length. Ovulation, which precedes fertilization by ≤24 hours, should give a more accurate estimate. Methods: The Effects of Aspirin in Gestation and Reproduction (EAGeR) trial preconceptionally enrolled participants from four US medical centers from 2006-2012. Participants in our analyses delivered a singleton live birth, had prospectively recorded LMP, ovulation detected by fertility monitor, and early first-trimester crown-rump length measurements. We estimated pregnancy length, preterm birth (<37 weeks) prevalence, and sex-specific size for gestational age by LMP, ultrasound, and ovulation. We report sensitivity and specificity of LMP and ultrasound for detecting preterm birth compared to our gold standard, ovulation. Results: In our analytic sample (n=392), pregnancies were longest, preterm birth least common (prevalence = 0.07, 95% CI: 0.04, 0.10), and small for gestational age most common when measured by LMP. Pregnancies were shortest, preterm birth most common (prevalence = 0.10 (95% CI: 0.07,0.13), and small for gestational age least common when measured by ultrasound. The prevalence of preterm birth was 0.08 (95% CI: 0.06, 0.12) by ovulation. Using ovulation as the gold standard measure, LMP was less sensitive in detecting preterm birth (0.76, 95% CI: 0.61, 0.90) than ultrasound (0.94, 95% CI: 0.86, 1.00). The specificity of LMP was 1.00 (95% CI: 0.99, 1.00), and the specificity of ultrasound was 0.97 (95%CI: 0.96, 0.99). Conclusion: While this study’s pregnancy length information is best-case scenario, we observed misclassification of outcomes that may inform future bias analyses.