Let's get the downside out of the way first. Yes, to be honest, the plot is rather thin and when the film has finished will inspire a fair amount of disbelief in those who care to exmaine the story.
But on the upside, whilst you are immersed in the fim's wonderful cinematography, its bleak pallette of wintery colours and grainy visuals, those long, still scenes, and the moody, hypnotic soundtrack, you will be sucked into the atmosphere of suspense and mystery.
Don't listen to all the naysayers, this film is well worth a watch. Lock Stock and 2 Smoking barrels it certainly isn't (as a 1 star reviewer unfairly compared it to). If that's you're kind of film then go and watch something else. If you want to explore a different kind of film making, one with style, substance and scenes of quiet contemplation, then watch Birth.