Well to go to those lengths, someone had some sort of mental instabilities...
Maybe OP "mom" was one of those women that told everyone she was pregnant, even knowing she wasn't. So when her "due date" rolled around, she had to do something to save face? The adoption process isn't quick, or easy in most countries?
She could of thought she was pregnant, found out she wasn't, still kept up the facade?
She was pregnant, lost the baby, then lost her mind?
Found out she could never get pregnant, so felt inadequate. Didn't want her family and friends to know, so instead of being a normal human being, and adopting legally, she went to an extreme. Faked the whole pregnancy, and stole the baby (babies)?
Maybe she did this same thing for other people, for profit. Figured she may as well do the same for herself?