Just for the record: I had a sim who officially became pregnant (i.e. spun into maternity clothes) about 15 minutes after her carpool showed up. This seemed to have gotten her stuck in a kind of quantum state where she was both supposed to be at work and not allowed to be at work, as well as hanging up the process of actually starting the pregnancy (the camera zoomed, the music played, she never changed clothes or got the "pregnant" moodlet). Since there were only about 6 game hours elapsed since my last save, and Metro has said in the past (to paraphrase) "if you have a bug, you may fix it, even if what you do to fix it would normally not be allowed", I reverted to the save file and had her call to take the day off work.
This resolved the work issue, but somehow it still took 90 game minutes between camera zoom/music and her actually changing clothes and getting the moodlet. I'm a little nervous about the pregnancy now.