Shares of Inc slumped 19 percent on Thursday, making them among the biggest percentage losers on the Nasdaq, after the company forecast slower growth in subscriber additions for 2012, its second consecutive year of decline.

The company, which operates a website that allows people to trace their family roots by scouring online records, expects to end 2012 with 12.4 percent to 14.5 percent more subscribers than 2011 end.

It grew subscribers by 22 percent in 2011 and 31 percent the year before. Fourth-quarter average monthly revenue per subscriber dropped sequentially by 30 cents to $18.38.

BMO Capital markets said lower ratings on the early episodes of the third season of NBC's Who Do You Think You Are, for which Ancestry provides research and brings in ads, have put pressure on the stock.

The celebrity genealogy show's first two episodes this season, featuring actors Martin Sheen and Marisa Tomei, recorded an average of 5.8 million viewers. This was 17 percent lower from the first two episodes of season 2 that aired last year, the brokerage said.

We believe the diminished impact of the show...will be partially offset by a longer season this year, BMO said in a research note and maintained its outperform rating on the stock. shares fell to $23.06, their lowest since January 4, after which they had received a boost when Ancestry came out with a strong revenue outlook for 2012.

More than 2 million shares changed hands by 10.35 ET, nearly twice their normal volumes.

(Reporting by Sayantani Ghosh in Bangalore; Editing by Gopakumar Warrier)