I am after some advice regarding a downrate of my performance rating. I came back after maternity leave in September. I had the performance review with my line manager in December and he told me that I would get the top rating "exceeding contribution". After a calibration exercise from the managers above him, I have been downrated to "successful contribution". I have asked him today for the reason of this and he mentioned that it is because I have been off for 8 months of the year. Can they do this or is it maternity discrimination?