Can an employer extend the probation period of a woman employee if she goes on maternity leave?

Yes, probation can be extended if her absence (including maternity leave) means you could not assess her performance, but it must not be discriminatory.
No, you cannot extend probation just because she availed maternity leave; it should be framed as a neutral extension due to insufficient time for review.

Shekar Ganagaluru's Books

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K. VITTALA RAO's HR Books