Because she is, apparently. While Palin continues to deny the allegations that her most recent child wasn’t hers, she has admitted that her daughter is pregnant and intends to marry the father- which explains a lot. This passes the smell test for me and so I believe any further investigation into the child situation will be unnecessary. I, for one, am convinced.
The Republican vice presidential candidate Sarah Palin has announced that her 17-year-old daughter is pregnant, in an announcement intended to knock down rumors by liberal bloggers that Palin faked her own pregnancy to cover up for her child.
Bristol Palin, one of Palin’s five children with her husband, Todd, is about five months pregnant and is going to keep the child and marry the father, the Palins said in a statement released by the campaign of Republican presidential candidate John McCain.
Hat Tip to Kate for the Link.
P.S.A.P. ... rock out with your wonk out. 
“Senator McCain has a long legislative record of supporting abstinence-based initiatives in his record in the U.S. Senate,” said Trey Walker, McCain’s South Carolina campaign director. “He thinks that abstinence is healthier and should be promoted in our society for young people.”
http://www.washingtonpost.com/wp-dyn/content/article/2007/02/16/AR2007021600978.html
Maybe little Bristol Palin should have been taught about birth control. Telling adults to simply keep their pants on, IMHO, has never worked to any great degree. Telling it to teens is even more ineffective. As the soon to arrive addition to the Palin family provides living proof of.
Note to Senator McCain – You may have thought Chelsea Clinton was ugly, but she had enough sense not to get pregnant before she was even out of highschool.