While parents Hal and Lois are engaged in a pregnancy scare, the episode flashes back to the births of all four boys. It is a sweet episode, or at least as sweet as this show gets.
The flashbacks are all great foreshadowing of how the kids will eventually turn out. We see the origins of Malcolm's genius, that Reese's cruelty seems to be a genetic defect since it is with him in the womb, and why perhaps Dewey is a little strange--don't stand too close to the microwave!
When the show does sweet, it does it very well. The episode never borders on sappy, but the genuine affection the family feels for each other is definitely present, even in their cruelest moments.