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In the recruitment phase, They don't need to know and by law are not allowed to ask for; date of birth, marital status, sexual orientation, family situation anything that they can base discrimination on.
That is so right, so I think you don't need to mention mat leave either. If I was recruiting and someone mentioned that they were seeking new employment while on mat leave I might think that was a bit "rude" or disloyal of them. But that is just me... And I know each person has special circumstances, for example old job not catering to your new working hour requirements. So if you are going to mention mat leave, maybe give the reason you are seeking alternate employment in your opening statement.