 According to the Nielsen ratings system , " Rosemary 's Baby " was viewed by an average of 6 @.@ 5 million American viewers . The episode achieved a 3 @.@ 1 / 8 in key 18 – 49 demographic . The 3 @.@ 1 rating refers to 3 @.@ 1 % of all 18- to 49 @-@ year @-@ olds in the U.S. , and the 8 share refers to 8 % of all 18- to 49 @-@ year @-@ olds watching television at the time of the broadcast . In the U.S. , " Rosemary 's Baby " was up by 19 % in the rating demographic compared to the previous episode , " The Collection , " receiving its highest result since the second season premiere episode , " <unk> , " on October 4 , 2007 . 
