 Insomnia is associated with negative outcomes in pregnant women, so it is important to assess the severity of insomnia. The Insomnia Severity Index, ISI, is a widely used tool to measure insomnia, however its factor structure and structural invariance for pregnant women has not yet been investigated. We conducted a cross-sectional study with the ISI in Japan, including 382 pregnant women aged 10 to 13 weeks. We found that the ISI had a two-factor structure, which was consistent across parity and time points. This suggests that the ISI can be used to measure insomnia in pregnant women, regardless of their parity or time point. This article was authored by Arikoshinohara, Ayako Hada, Mariko Minitani, and others.