It was the teenage pregnancy that made international headlines and for a moment threatened to derail Sarah Palin’s vice-presidential bid, but the weekend birth of the Governor of Alaska’s first grandchild almost went unnoticed.

Bristol Palin, the 18-year-old daughter of the controversial female Republican candidate, gave birth to a 7lb 4oz baby boy, Tripp Easton Mitchell Johnston, on Saturday in Palmer, Alaska.

Colleen Jones, the sister of Bristol's grandmother Sally Heath, told People magazine: “We think it's wonderful. The baby is fine and Bristol is doing well. Everyone is excited."

Baby Tripp, who was reportedly due on December 18, takes the surname of his 18-year-old father, Levi Johnston, who said last month he plans to marry Bristol sometime in 2009.

Bristol is the oldest daughter of Sarah and Todd Palin’s five children – they have two other daughters Willow and Piper, and two sons, Track (who is currently serving in Iraq) and Trig, an infant who has Down’s syndrome.

The teenager’s pregnancy made worldwide news when it was revealed in September, just days before her mother’s acceptance speech to the Republican party to be John McCain’s running mate for the US election, which was won by Democratic president-elect Barack Obama.

In September the Palins said of their daughter’s pregnancy: “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."

“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.”

In October Mr Johnston, a keen hunter and self-confessed ‘redneck’ who has a tattoo of Bristol’s name on his ring finger, revealed the couple had planned to marry before she became pregnant.

"We were planning on getting married a long time ago with or without the kid,” he said.

"That was the plan from the start.

"We both love each other. We both want to marry each other. And that's what we are going to do."

Of the baby, Mr Johnston said: "We're up for it. I'm excited to have my first kid. It's going to be a lot of hard work but we can handle it."

The young couple are currently based in the Palin family hometown of Wasilla, Alaska, and are both studying – Bristol is completing her high-school diploma through correspondence courses, while her fiancé is studying to become an electrician.

Mr Johnston's mother, Sherry, was arrested on drug charges earlier this month after an undercover narcotics investigation.

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