BACKGROUND: Previous studies of prenatal acetaminophen use have not addressed what indications and maternal co-factors describe acetaminophen use. OBJECTIVE: The objective of this study was to describe these parameters in a well-characterised, prospective birth cohort. METHODS: Data were drawn from the MotherToBaby study of pregnant women enrolled from 2004 to 2018. Daily acetaminophen diaries were calculated for all exposed women with complete dose and duration information. Descriptive statistics were used to assess maternal characteristics associated with acetaminophen use. Prevalence by 2-year interval was described, and linear regression was used to test for trend. Indication of use and dose per indication were summarised. RESULTS: Of 2441 subjects, 1515 (62%) reported use of acetaminophen. Over the 15-year period, there was a decline in use of 2.5% for each 2-year period (test for trend = 0.001) with 58% reporting acetaminophen use in 2017-2018. Among women with acetaminophen use in pregnancy (n = 1515), 58% reported <10 days of use, 13% reported 10-19 days of use, 9% reported 20-44 days of use, and 9% reported 45 or more days of use. Twelve per cent had undefined duration of use. Increasing duration of exposure was associated with tobacco use, obesity, self-reported depression or anxiety, and antidepressant use. The most frequently reported indication was headache, however, indication varied by duration of use, with more women reporting use for sleep or pain/injury in the categories with the longest duration of use. Median dose per exposed day was highest among those reporting use for sleep, and higher doses were more frequently reported for arthritis, injury, and pain. CONCLUSION: Acetaminophen is used by the majority of pregnant women, and some continue to use for many weeks in pregnancy. Given the heterogeneity in duration of use, indication, and dose, studies that estimate the risk of adverse outcomes associated with acetaminophen must carefully consider these factors.