Prenatal opioid exposure is an established public health problem, in particular among Medicaid-covered births. Yet, existing prevalence rates are plausibly underestimated. We leverage extensive linked longitudinal administrative data for all Medicaid-covered live births in Wisconsin from 2010 to 2019 to estimate a range of prevalence rates using an innovative strategy that jointly accounts for both likelihood of exposure and potential risk to prenatal development. We find that 20.8% of infants may have been prenatally exposed to opioids, with 1.7% diagnosed with neonatal abstinence syndrome and an additional 1.2% having a high combined likelihood of exposure and potential risk to prenatal development, 2.6% a moderate combined likelihood and risk, and 15.3% a low or uncertain combined likelihood and risk. We assess improvements in prevalence estimates based on our nuanced classification relative to those of prior studies. Our strategy could be broadly used to quantify the scope of the opioid crisis for pregnant populations, target interventions, and promote child health and development.