To examine trends and correlates of frequency of self-reported alcohol and nicotine use among pregnant women. Cross-sectional study of 363,240 pregnancies from 2009 to 2017 screened for self-reported substance use at their first prenatal visit in Kaiser Permanente Northern California. Poisson regression with a log link function was used to estimate the annual prevalences of self-reported daily, weekly, and ≤ monthly alcohol and nicotine use, adjusting for socio-demographics. Generalized estimating equation models were used to estimate the adjusted odds ratios (aOR) of any self-reported prenatal alcohol or nicotine use among those who self-reported use in the year prior to pregnancy, by frequency of pre-pregnancy substance use and socio-demographics. The sample was 64 % non-White [mean (SD) age = 30.1 (5.6)]. From 2009-2017, alcohol use before pregnancy increased from 63.4%-65.9% (trend p-value = .008), and prenatal alcohol use decreased from 11.6%-8.8% (trend p-value<.0001). Nicotine use before pregnancy decreased from 12.7 % to 7.7 % (trend p-value<.0001), and prenatal use decreased from 4.3 % to 2.0 % (trend p-value<.0001). Trends by use frequency were similar to overall trends. The odds of continued use of alcohol and nicotine during pregnancy were higher among those who used daily or weekly (versus monthly or less) in the year before pregnancy and varied with socio-demographics. Prenatal alcohol and nicotine use decreased from 2009 to 2017. More frequent pre-pregnancy use predicted higher odds of prenatal use. Results suggest that interventions and education about the harms of prenatal substance use for frequent users prior to conception may reduce substance use during pregnancy.