Increased adverse pregnancy outcomes with unreliable last menstruation

To estimate the risk of adverse outcomes in women whose first day of the last menstrual period (LMP) was unreliable.

Among 20,244 singleton pregnancies with measurements of biparietal diameter between 12 and 22 weeks' gestation, LMP was registered as unreliable in 3775 (18.6%) and reliable in 16,469 (81.4%). Adverse outcomes were defined as spontaneous or missed abortions after 12 weeks' gestation, stillbirth or postnatal death within 1 year, preterm birth, birth weight less than 2500 g, and low birth weight (LBW) for gestation (lower than 22% below sex-specific expected weight). Logistic regression analysis and Kaplan-Meier survival analysis were used to analyze the risk of adverse outcomes.

The risk of death was doubled in pregnant women with unreliable LMPs compared with those with reliable LMPs (odds ratio [OR] 2.0; 95% confidence interval [CI] 1.5, 2.6). This risk was highest with respect to stillbirth (OR 2.7; 95% CI 1.7, 4.3). The risks of preterm birth, LBW, and LBW for gestation were also significantly increased (ORs 1.5, 1.4, and 1.2; 95% CIs 1.3, 1.7; 1.2, 1.6; and 1. 0, 1.4, respectively).

An unreliable LMP is associated with increased risk of adverse outcomes, especially fetal death.