Stephanie Zacharek is explicitly comparing Knocked Up to Preston Sturges comedies.
That uncertainty is what links it to the great American romantic comedies: It's not as elegant as, say, "Holiday" or "The Lady Eve" or "The Palm Beach Story," but it's wise enough to know that the false promise of happily ever after is more depressing than it is uplifting. Better to acknowledge the bumpiness of the road ahead than to fool yourself into believing you can iron out its kinks.
I think she's dead-on here. Knocked Up is too raunchy to work like The Lady Eve or The Palm Beach Story, both of which were sex comedies of a different sort, but it shares the sex-with-consequences sensibility of The Miracle of Morgan's Creek with the broad pleasures of 80s sex comedies (I'm thinking Porky's), but also with a grown-up take on parenthood that I can't recall seeing in any movie before. There's a very warm embrace of humanity in the movie that reminds me of the greatest humanist director, Renoir, specifically the hijinks of Boudu Saved From Drowning. I think that's where I am: half Preston Sturges, one quarter Porky's, and one quarter Renoir.
I don't want to ruin anything for anyone, but since it's a comedy about pregnancy, I'm going to assume that y'all know it winds up in a delivery room. That scene was just incredible, somehow combining slapstick with the very real confusion and beautiful grossness (by which I mean everyone is born in blood and struggle) of natural childbirth. It has an unwavering belief in the realness and decency of even the most minor of supporting cast, and the overall effect is profound. I'm a sap these days, I know, because when my family is expecting a baby, any images of childbirth cut straight to my weepy emotional place, and that's exactly what this incredible scene does.
There's several other points where the character's reality are realized in a way that few comedies could handle. My favorite is a moment where Paul Rudd's character, holding a ridiculous fairy-tale castle-shaped ice-cream cake for his daughter's birthday, learns what his wife and sister-in-law really think of him, and in, like, three seconds, he registers the incredible hurt of this and moves on. If the movie weren't so insanely funny and light on its toes, it could easily play like an agonizingly detailed examination of marriage and relationships. That's a rare and awesome thing.