"Bloggers had infuriated the McCain campaign over the weekend after airing rumors that Palin's 4-month-old son was actually her daughter's baby.
McCain campaign officials said Bristol's pregnancy was made public in order to counter what one aide called "mudslinging" that has circulated on blogs like Daily Kos.
According to postings on Daily Kos, Sarah Palin allegedly faked a pregnancy and pretended to give birth in April to Trig, who has Down syndrome.
The McCain campaign said Bristol Palin is five months pregnant and that Trig was born on April 18, making the rumor's timing impossible.
Palin would be due just a few weeks before Inauguration Day in January.
In a posting on Sunday, Daily Kos said, "Sarah needs to honestly answer for her lies - promptly." "