Use of marijuana, an antiemetic, is increasing among pregnant women,1,2 and data from 2 small surveys3,4 indicate that women self-report using marijuana to alleviate nausea and vomiting in pregnancy (NVP). To date, only 1 epidemiologic study5 has examined whether women with NVP are at elevated risk of using marijuana. The study of 4735 pregnant women in Hawaii5 from 2009 through 2011 found that self-reported prenatal marijuana use was more prevalent among those with (3.7%) vs without (2.3%) self-reported severe nausea during pregnancy. We used data from a large California health care system with standard universal screening for prenatal marijuana use via self-report and urine toxicologic tests from January 1, 2009, through December 31, 2016, to test whether prenatal marijuana use is elevated among females with a diagnosis of NVP. Methods:Kaiser Permanente Northern California (KPNC) is a multispecialty health care system serving more than 4 million members representative of the Northern California area. The sample consisted of pregnant females 12 years or older in KPNC who completed a self-reported substance use questionnaire and urine toxicologic test in the first trimester (at approximately?8 weeks gestation) during standard prenatal care. All positive toxicologic test findings were confirmed with a laboratory test result. The institutional review board of KPNC approved this study and waived the need for informed consent. Nausea and vomiting during the first trimester of pregnancy (90 days from last menstrual period) was based on International Classification of Disease, Ninth Revision, Clinical Modification, and Tenth Revision, Clinical Modification, diagnostic codes in the electronic health record and categorized as severe (hyperemesis gravidarum [codes 643.00, 643.03, 643.10, 643.13, O21.0, and O21.1]), mild (other NVP diagnoses [codes 536.2, 643.80, 643.90, 643.93, 787.01, 787.02, 787.03, G43.A0, O21.9, R11.0, R11.10, R11.11, and R11.2]), or none. We estimated the adjusted odds of prenatal marijuana use among females with NVP using multilevel logistic regression, controlling for age, race/ethnicity, median neighborhood household income, year, and self-reported marijuana use in the year before pregnancy from the universal screening questionnaire for prenatal substance use. We used in the PROC GLIMMIX procedure in SAS software (version 9.3; SAS Institute, Inc) for all analyses, and 2-sided P?