ST. PAUL, Minn. -- Alaska Gov. Sarah Palin announced Monday that her 17-year-old daughter, Bristol, is five months pregnant and plans to have the baby and marry the father.

"Our beautiful daughter Bristol came to us with news that as parents we knew would make her grow up faster than we had ever planned," Palin, the presumptive Republican vice-presidential nominee, said in a statement issued by Sen. John McCain's presidential campaign. "We're proud of Bristol's decision to have her baby and even prouder to become grandparents. As Bristol faces the responsibilities of adulthood, she knows she has our unconditional love and support."

The announcement comes less than a week after McCain introduced Palin, casting her as an all-American mother with five children. Her biography -- which includes her giving birth to a son, Trig, with Down syndrome in mid-April -- has been a touted as a key part of her appeal, especially to conservatives.

Almost since the day Sarah Palin was named as McCain's running mate, rumors have circulated on liberal blogs that raised questions about whether Bristol Palin was Trig's mother.

The statement Monday appeared designed to rebut that rumor. The McCain campaign confirmed that the daughter is five months pregnant, a timeline that, if accurate, would make it almost impossible for her to have delivered a baby in April.

Senior McCain adviser Steve Schmidt said the campaign decided to issue a statement about Bristol Palin's pregnancy to help quell those Internet-based rumors, which he called "disturbing, nasty smears."