Flowchart showing enrollment in nuMoM2b sleep-disordered breathing substudy and inclusion in analysis. *For the 728 women (3,550 minus 2,822) without sleep studies in mid-pregnancy, 30 had a preterm birth before the sleep study could be completed, 86 missed the study visit (including the sleep assessment), and 612 had the study visit but did not participate in the mid-pregnancy sleep study. In planning the study, it was assumed that 25% would refuse the second sleep study. The participation rate in the mid-pregnancy sleep study was higher than expected. nuMoM2b, Nulliparous Pregnancy Outcomes Study: Monitoring Mothers-to-Be; IQR, interquartile range.