Meredith Baxter Birney does not appear in some episodes in Season 3 (fall 1984) because she (while playing a pregnant mother) was herself pregnant and would deliver twins on 10/2/1984.  Here are the episodes:

Incidentally, she did appear in "Don't Kiss Me, I'm Only the Messenger" (production code 053), though only briefly at the beginning.  (Based on the production codes, one might have expected her to miss that one).

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