• For any month’s coverage estimate, the denominator is the number of persons with a pregnancy during the current flu season (defined as August through March) beginning before or during the specified month. The numerator is the subset of the denominator who have received flu vaccination prior to, during, or after pregnancy. The denominator increases as more persons are identified as pregnant or having been pregnant during the flu season. Cumulative vaccination coverage for one month may be lower than cumulative coverage for a previous month due to addition to the denominator of persons who are less likely to have received vaccination.