Bristol Palin appeared on stage with her parents in Ohio last week
Alaska Governor Sarah Palin, newly picked as Republican John McCain's running mate, has revealed that her 17-year-old daughter is pregnant.
Mrs Palin, a social conservative who is opposed to abortion, said in a statement her daughter Bristol would keep the child and was to get married.
The Alaska governor, a mother-of-five, was presented three days ago as her party's vice-presidential candidate.
The news comes as the Republicans' national convention gets under way.
The McCain campaign and Palin family asked for respect for the young couple's privacy.
Mrs Palin and her husband Todd said in a statement: "Bristol and the young man she will marry are going to realise very quickly the difficulties of raising a child, which is why they will have the love and support of our entire family."
Barack Obama: This story has no relevance
Bristol is reportedly five months pregnant and would be due to give birth in late December.
Advisers to the McCain campaign said they had known about the pregnancy before offering Mrs Palin the vice-presidential nomination.
"Senator McCain's view is this is a private family matter," said McCain spokesman Steve Schmidt.
"As parents, [the Palins] love their daughter unconditionally and are going to support their daughter. Life happens."
Democratic presidential candidate Barack Obama, asked for his response to the news while campaigning in Michigan, said people should "back off" from such stories.
"I think people's families are off-limits, and people's children are especially off-limits," he told reporters.
"This shouldn't be part of our politics. It has no relevance to Governor Palin's performance as a governor, or her potential performance as a vice-president."
The announcement of Bristol's pregnancy followed rumours on internet blog sites over the weekend suggesting that Mrs Palin's youngest child, Trig, born in April, had actually been born to Bristol. The boy has Down's Syndrome.
The National Journal reported that a spokesman for Mrs Palin had also confirmed that her husband had been arrested for driving under the influence in 1986.
The news of Sarah Palin's daughter's pregnancy is gut-wrenching for Republicans
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