The episode ‘If …’ calls on that trusty old trope of a traumatic event – in this case a plane crashing into Wisteria Lane – causing various characters to reflect on what might have been had they made different life-choices. Carlos reflects on how Gaby would be as an over-protective mom; Lynette thinks about a future with her unborn twins; Susan contemplates a life with Karl as an overweight unhappy housewife had she not left him.
Susan imagines that she gave Karl another chance after discovering his infidelity. The stress of her decision consumes her, and she gains weight. Eventually she throws her not-inconsiderable self at the local plumber, Mike Delfino, in an attempt to even the score with Karl. In the end though Karl leaves her even after she loses the extra pounds. In the final scene, an abandoned Susan watches as Mike Delfino escorts a pregnant blond lady into their home.
John M Elliott Jr was the head of the makeup department; Matthew Mungle (W.M. Creations inc) was responsible for the special makeup effects. I understand that Koji Ohmura worked on the makeups though uncredited.