Background Regulatory agencies in the United States (US) and Europe differ in requirements for defining pregnancies after the last dose of oral contraceptive, sometimes resulting in discrepant Pearl Indices (PIs) for the same product despite identical clinical data. This brief report highlights one such example, a 91-day extended-regimen combined oral contraceptive (COC). Methods The US- and European-based PI methodologies were compared for a 91-day extended-regimen COC consisting of 84 days of active levonorgestrel/EE 150 μg/30 μg tablets, followed by 7 days of EE 10 μg tablets in place of placebo. Conclusions At the times of approval of the 91-day extended-regimen COC in the US and Europe, the requirements for defining ‘on-treatment’ pregnancies differed (14-day vs. 2-day rule, respectively). This difference resulted in a higher PI in the US- vs. European-based calculation (1.34 and 0.76, respectively). The differences in the PI should not be interpreted as the extended-regimen COC being less effective in preventing pregnancy in the US compared with Europe.