The very notion of Arnold Schwarzenegger getting pregnant - although posing some uncomfortable biological questions - may not, given the advance of science, be such an impossible idea. And it certainly makes for a fascinating movie prospect.
Junior is so "high concept" it could have snow on it. In fact, you can almost hear the studio exec being pitched by some hopeful scriptwriter: "Arnold gets pregnant." Herein lies the problem, for were it just an Ordinary Joe rather than the bicepular-one who suddenly found himself nurturing a sprog, you feel there would have been more scope for the expected gender-swapping hilarity.
As it is, the laughs are gained not so much from the idea of man-as-mother, as largely at the expense of the Sperminator playing against type. Director Reitman also fails to resist harping back to the big-concept of his earlier Twins, with numerous sight gags based on the physical disparities 'twixt the male leads.