Most women take medication during pregnancy despite limited scientific evidence on safety. We investigated medication use, including changes in and reasons for changes in use during pregnancy, with attention to medication use in pregnant women with chronic conditions. We conducted an online survey of pregnant women aged ≥18 years ( = 1,226). We calculated descriptive statistics for aspects of medication use and performed multivariable logistic regression to examine associations between change in use and chronic conditions. Seventy-nine percent of women took at least one medication during pregnancy. Among those, 63.2% made at least one medication change: 42.0% started, 34.9% stopped, 30.0% missed dose(s), and 18.1% lowered dose(s) from that originally prescribed or recommended. More than a third (36.5%) of women who stopped, lowered, or missed medication did so independent of health care provider advice; 54.0% cited concern about birth or developmental defects as reasons for change. Odds of medication change were higher for women with chronic conditions: digestive conditions-starting (adjusted odds ratio [AOR] = 1.8, 95% confidence interval [CI] = 1.1-2.7), stopping (AOR = 2.1, 95% CI = 1.4-3.3), and lowering (AOR = 2.4, 95% CI = 1.7-3.3) medication; mental health conditions-starting (AOR = 1.6, 95% CI = 1.2-2.2), stopping (AOR = 3.0, 95% CI = 2.3-4.0), or missing (AOR = 2.1, 95% CI = 1.6-2.8) medication; pain conditions-stopping (AOR = 2.9, 95% CI = 2.0-4.2); and respiratory conditions-starting (AOR = 2.0, 95% CI = 1.3-3.1), stopping (AOR = 1.7, 95% CI = 1.1-2.6), and missing (AOR = 2.2, 95% CI = 1.4-3.4) medication. Most pregnant women take medication and many, including those with chronic conditions, change their medication use during pregnancy. Medication change may occur independent of health care provider advice and due to women's safety concerns.