Can my employer fire me if I am pregnant?

No. During your pregnancy and up to four months after the birth (in case of a miscarriage, until after the 12th week of pregnancy), you are protected from termination.  

Termination is only permitted in very exceptional cases before or after the birth under very special circumstances. If you receive a termination during this time, it is important to seek advice as quickly as possible in order to take any possible legal steps against the termination. 

In this case, speak to a pregnancy counselling agency or a solicitor of your choice.