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If your wife could prove that this was being done to her specifically in retaliation for having taken pregnancy leave, that would be illegal. But I think the question is one of damages. No real damages are suffered as a result of not getting more notice, and without damages there is nothing to pursue through a lawsuit. Your wife would have to miss the shift and then argue that she could not have reasonably attended due to short notice, and then make the retaliation argument with her damages being loss of a job. But she would be essentially trading her job for a lawsuit, and that's never something you want to willingly do, as lawsuit are extremely stressful and generally take a year or more to resolve.
If your wife believes that this short notice is retaliatory and that her employer is motivated to continue retaliating against her, she should document all such instances of retaliation. If they continue, and especially if they become more severe, there might be enough to warrant a lawsuit at that point in time. At this point, however, legal action would be premature and would cause for more problems for your wife than it would solve.
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