Aims: No study has evaluated the BET pharmacokinetics in twin pregnancies separated by chorionicity. The aim this study is to describe and compare the BET pharmacokinetic parameters in singleton, dichorionic (DC) and monochorionic (MC) twin pregnancies in the third trimester of pregnancy. Methods: Twenty-six pregnant women received an intramuscular dose of 6 mg of BET sodium phosphate plus 6 mg BET acetate. Serial blood samples were collected for 24 hours after the first intramuscular BET esters dose. BET plasma concentrations were quantified using a validated HPLC analytical method. BET pharmacokinetic parameters were obtained employing a non-compartment model, and were compared using ANOVA’s test with Tukey’s multiple comparisons test. Correlations between clinical features and pharmacokinetic parameters were analyzed using Pearson’s correlation. Preliminary data on the BET placental transfer were also presented. Results: The geometric mean (IC 95%) of AUC0-∞ 670.0 (504.3-805.2) vs 434.9 (311.2-539.6) ng.h/mL and the CL/F 18.38 (13.84-22.65) vs 29.40 (21.17-36.69) were significantly lower and higher, respectively, in DC twin pregnancies compared to singleton. Others pharmacokinetic parameters did not differ among the groups. Conclusions: Data from this study suggest that the presence of two fetoplacental units may increase the BET metabolism by CYP3A4 enzyme and increase its elimination. Pharmacokinetic-pharmacodynamic clinical studies are needed to investigate whether this BET pharmacokinetic changes have clinical repercussions for the newborns and require dose adjustment in DC twin pregnancies.