It is a very rare event that the first mid-term after a new party enters the White House DOESN'T go largely against the party in power.
Since FDR, only twice has the first mid-term gained seats with the party in power - FDR in 1934 and Bush in 2002. And that's not surprising; FDR was being credited with ending the Depression, and Bush had entered us in a war against the presumed perpetrators of 9/11.
And the timing of all the others - not so good. Poor Gerry Ford saw a huge loss - right after Nixon resigned. Even Reagan saw a loss, because we were still in recession.
One of the reasons this happens is what is sometimes termed the coattail effect - a new party comes into the White House, and the popularity of the candidate at the top of the ticket has enough clout to swing voters in "purple" districts. A new wave of newly minted House members win in districts where their share of the vote is fairly thin - but after a couple years, it snaps back.
Sometimes, in the case of Clinton and Obama, the blowback from voters is very strong, and the voters showing up consist of concerned and motivated members of the opposition. The candidate in the White House has pissed off a lot of voters early on rushing into disliked legislation, and they get them riled up enough to show up in numbers. Mid-terms tend to be that way - it's always been that voters don't show up as much, but they do when it's the party they oppose.
Pelosi is presiding over I think the narrowest margin in the House in history - and she thinks she'll keep it? A smart thing to say would be she doesn't know, or doesn't trust polling - if you're the Speaker of the House, you don't go around saying you think you'll lose, even though it's almost a certainty, and there's good reason to believe that the bloodletting will be WORSE, as Republicans are gaining ground in blue areas.
But it's sheer stupidity to declare you'll win. Not unless she knows something the rest of us don't know.