I can't find this one at the Vault and I think I'd remember this cover. "Thomas Altman" is a pseudonym used by Campbell Black, whose other novels include Kiss Daddy Goodbye (1980), Black Christmas (1983), Dark Places (1984) and The Intruder (1985).
Whether Black Christmas is a delayed tie-in with the film of that name, I don't know.
From the cover:
"Something is happening to Ellen.
"Ellen Campbell is eight months pregnant and happily looking forward to the birth of her first child - until strange things start happening. A cherished possession is missing. A treasured object has been ruined. And love, the most valuable possession of all, is under attack.
"Suddenly, everyone Ellen ever trusted is under suspicion as she races against time to discover who is trying to kill her and her unborn child..."
"A well-done narrative of paranoia à la Rosemary's Baby." Publishers Weekly