Methods
Patients A cross-sectional pharmacokinetic study of singleton and twin pregnancies was performed, including pregnant women underwent tocolysis in the local hospital because of preterm labour from August 2018 to September 2019. This study was approved by the local Ethics Research Committee (protocol number 8676/2017) of University Hospital of the Ribeirão Preto Medical School (São Paulo, Brazil), and patients gave signed informed consent. The study protocol was registered on the Brazilian Registry of Clinical Trials (ReBEC, www.ensaiosclinicos.gov.br) under ID number RBR-2v2mwkz. Participation in the study was limited to the inpatient population to ensure compliance with serial blood sampling. Thirty-five pregnant women, between 24 and 34 weeks of gestation, and >18 years old were clinically eligible for the antenatal corticosteroid treatment and were approached for enrollment. The following exclusion criteria were applied: use of tobacco, alcohol or illegal substances; maternal diseases with vasculopathy identified by laboratory tests or signals of placental insufficiency; the use of drugs that interact with BET; or maternal desire to withdraw. Finally, the data from 26 pregnant women, divided into three groups (9 singleton pregnancies, 9 DC and 8 MC twin pregnancies) were analyzed.