In contrast, Chou and colleagues (2008) conducted a prospective longitudinal study of 91 pregnant women using the index of Nausea, Vomiting, and Retching measure (INVR) and reported the second and third trimesters to be associated with significantly lower levels of NVP than in the first trimester, even though mean INVR scores for the second trimester did not significantly differ from the third trimester. Diffences between our findings and those of Chou et al. might be explained by our inclusion of women in both the first and second trimester.