In Lyons v DWP JobCentre Plus the EAT held that dismissal for absences due to depression arising after maternity leave do not amount to sex or pregnancy discrimination under section 13 or 18 of Equality Act 2010.
Lyons suffered post-natal depression. By being dismissed she was treated unfairly for a pregnancy-related illness however it only amounts to discrimination under s.18 if it occurs in the protected period. The protected period you will remember is the period from the beginning of pregnancy and the end of maternity leave. Since she was dismissed after the end of the protected period her claim under s.18 did not succeed.
Lyons’s claim for s.13 direct sex discrimination also failed. Following Brown v Rentokil the EAT held that if a woman suffers a pregnancy-related illness after her maternity leave has ended her employer is entitled to take into account the period of absence after the end of mat leave and compare that period with any period of sickness in a man.
It seems to me that Ms Lyons might have had better prospects of success in presenting a disability discrimination claim (subject to meeting the s.6 definition of disability) rather than seeking to tweak the sex/mat/pregnancy provisions of Equality Act 2010 beyond breaking point.