The trope that "mom wants another baby and dad flies into a tizzy" is a well-worn staple of sitcoms. In fact, it's so common that I could have sworn The Simpsons had done it in the past. But I guess not. I guess I must have been thinking of the thousands of other "mom wants another baby" episodes.
"Adventures in Baby Getting"
Yet another "Mom wants another baby" plot
It even retreads fairly specific territory. Near the end, Homer and Marge decide that their last resort is the sperm bank where Homer secretly made hundreds of deposits over the years. This is also the last resort for Francine in the American Dad "mom wants another baby" episode. It's not the episode you want to invite comparisons to, either, because American Dad really pulled out all the stops. Just thinking about the cartoon mascots at the vasectomy lab makes me giggle.
This episode had some truly funny moments (as when Skinner describes the school as "a hurricane shelter with voting booths," or M. Night Shyamalan's Flop House.) It's just a pity that they were in the service of such a pedestrian plot. The voice talent really proved their chops in this episode, though; Dan Castellaneta's read on Homer's "An ON PURPOSE baby?" absolutely slays me.
I also thought that the episode really missed out by not making a callback to the famous Canyonero. (It's a squirrel-smashin' deer-smackin' drivin' machine!) The whole thing starts when Marge has to shop for a new car; why not even mention the Canyonero? Never mind that the thin excuse for Marge's baby fever is that her new car won't fit a fourth child. A problem that the Canyonero (which smells like steak and seats thirty five) would not have.
You know what I would like to see some day? An "aging parent wants a new baby" episode where it's dad who wants a new child. It's always the mom who wants a new baby, the same way it's always mom who goes on strike, and mom who does all the nagging, and a thousand other sexist tropes embedded so deeply in our culture that I fear our ability to ever dig them out.
In the B plot, Lisa leads the boys on a merry chase after the mystery of her after-school disappearances. This plot had little intrigue for me, since I figured it out the instant I saw Lisa's dropped note. Maybe I spend too much time on font download websites, where oddball "uses all the letters" phrases are the norm. The AV Club review nailed it with the comment that this plot is "not any funnier than it sounds."