Palin's daughter stood holding her 5-month old brother Trig when her mother accepted McCain's offer to be his running mate.
Hoover Institution research fellow Bill Whalen says it is too soon to gauge the political fallout.
"This is certainly a surprise. The question is, are there any more surprises in the future or is this it and we just move on to her speech on Wednesday and the rest of the campaign, or is she going to be bogged down in a series of little surprises like this which could ultimately impair her candidacy," said Whalen.
Campaign aides say McCain knew about the pregnancy before he chose Palin and they only decided to announce it now because of speculation on liberal Web sites that Trig is really Bristol's baby. So the family confirmed she is about five months pregnant and will marry the teenage father.
In a statement, Governor Palin and her husband said, "As Bristol faces the responsibilities of adulthood, she knows she has our unconditional love and support."
Whalen says some on the left will say the pregnancy reveals conservative hypocrisy at its worst, and some on the right will say it proves the abortion rights opponent is true to her policies.
"So I suspect what matters here most is how it plays in the middle, and I've got a hunch that most people say this kind of thing happens to people in this day and age, it's a complicated world, people lead complicated lives and folks have to deal with situations like this," said Whalen.
Senator Barack Obama was empathetic.
"My mother had me when she was 18, and how family deals with issues and teenage children, that shouldn't be the topic of our politics," said Obama.
ABC News says a raft of Republican operatives and lawyers are just now on the way to Alaska to get ahead of the investigative reporters.
