OBJECTIVE: The purpose of this study was to assess whether maternal prepregnancy body mass index was associated with the use of antiemetic drugs in early pregnancy and/or with the occurrence of hyperemesis gravidarum. STUDY DESIGN: A retrospective, population-based, cohort study. Women who delivered singleton infants (n = 749,435) from 19952003 were evaluated concerning the use of antiemetic drugs in early pregnancy (data available from 1995). Women who delivered singleton infants (n = 942,894) from 1992-2001 were evaluated concerning hospitalization because of hyperemesis gravidarum (data available until 2001). Adjusted odds ratios were determined by Mantel- Haenszel technique and were used as estimates of relative risk (RR). RESULTS: Underweight pregnant women were more likely to use antiemetic drugs (RR, 1.19, 95% CI, 1.14-1.24) and to become hospitalized for hyperemesis gravidarum (RR, 1.43, 95% CI, 1.33-1.54) compared with ideal weight women. Obese women were less likely to use antiemetic drugs (RR, 0.93, 95% CI, 0.89-0.97) and less likely to require hospitalization because of hyperemesis (RR, 0.90, 95% CI, 0.85-0.95) compared with women with an ideal body mass index. CONCLUSION: The use of antiemetic drugs and the occurrence of hyperemesis gravidarum are related to maternal body composition.