Pregnancy Length Measurement Error: A Comparison of Last Menstrual Period and Ultrasonography with Ovulation-based...

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.

The Effects of Aspirin in Gestation and Reproduction (EAGeR) trial preconceptionally enrolled participants from four US medical centers from 2006 to 2012. Participants in our analyses delivered a singleton live birth, had prospectively recorded LMP, ovulation detected by a 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 the sensitivity and specificity of LMP and ultrasound for detecting preterm birth compared with our gold standard, ovulation.

In our analytic sample (n = 392), pregnancies were longest, preterm birth was least common (prevalence = 0.07, 95% confidence interval [CI]: 0.04, 0.10), and small for gestational age was most common when measured by LMP. Pregnancies were shortest, preterm birth was most common (prevalence = 0.10, 95% CI: 0.07, 0.13), and small for gestational age was 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).

While this study's pregnancy length information is the best-case scenario, we observed misclassification of outcomes that may inform future bias analyses.