Project summary Approximately 65% of US pregnant women take acetaminophen (in the US, branded as Tylenol; in Europe, paracetamol). Despite increasing concerns for its safety in pregnancy, with reported liver and developmental toxicity and risks for adverse child outcomes including hyperkinetic disorders and autism, it remains widely used and continues to be endorsed by US physician groups and the US FDA. It is well-established that pregnancy intake of medications can have carcinogenic impacts on offspring, as shown by diethylstilbestrol (DES) and vaginal adenocarcinoma. Yet potential carcinogenic impacts from acetaminophen are understudied. This study will be conducted via linkage of national health insurance claims databases in Taiwan, including the Taiwan Maternal and Child Health Database (which includes the Pharmaceutical Register and the Birth Registry, among other sources) and the Cancer Registry. Data on demographic, gestational, and other factors will be available from other national registers. The Aims of this study are to investigate the role of acetaminophen in cancer risk, addressing needs voiced by regulatory agencies, for more thorough research to better understand the safety of acetaminophen in pregnancy and to inform regulatory recommendations.