my goodness stick to your guns, your employer cannot under any circumstances get rid of you, and they should be f=doing a health and safety in pregnancy report through-out your pregnancy from the start, ask if they have done this, if they havnt demand that they do and if they refuse to put you on days see your gp and midwife to have aletter done to say you are deemed unfit to work nights given your history and they must adhere to this. dont back down as bad a you may feel, i went to hell and back when i was pregnant with my 2nd, my boss hated me anyway moreso when i told her i was preg,
had me in on disciplinaries for the time i had to take due to sickness, (i only had to move and vomited) and as it was a care home still had me lifting and working alone in a nursing wing, disgracful is only one of the words i will type to say how i feel bout them now! i had to start maternity early on orders of my midwife (10wks) as i was so big and she thought i might go early, specially with the work i was doing, my son was 8 days late and weighed 10lb 10ozs.
moral of the story dont let them treat you like dirt cos they will becos your pregnant and it messes up their rotas etc
hope it all works out, if in doubt speak to citizens advice solicitor