If you are treated negatively because of sickness absence that happens after your maternity leave has ended, this will be sex discrimination, unless your employer can prove that a man with a similar sickness absence record would have been treated the same. In any event, it is against the law to take into account any pregnancy-related sickness absence that predated the start of your maternity leave.
Some pregnancy-related conditions, such as severe post-natal depression, can amount to a disability, triggering protection from disability discrimination, including a duty to make reasonable adjustments, which, in some circumstances, could include adjusting sickness absence triggers.
If you do not return to work at the end of your maternity leave due to sickness absence, you should be paid sick pay under your employer’s standard sickness absence procedure. Make sure you meet all the requirements of the procedure as to notice and providing GP certificates. Ask your GP to state clearly on the GP certificate that your continued absence is linked to childbirth.
Take urgent advice from your union rep if you have one, or contact a specialist charity such as Maternity Action or a legal adviser.