To be honest, that's a management problem, not a you problem! They should have a back-up plan for their employees being off sick
@Deanne It’s so frustrating because if I’m the only one there and I go home etc then it affects the day to day running of the business, they took me on knowing I was pregnant and would be going on mat leave shortly after joining so I don’t want them to feel I’m taking the piss if that makes sense
Sounds more like if you feel you're forced to work despite not being up to it then they're taking advantage. You should have a discussion at least with your line manager and let them know how you're feeling, and ask what accommodations they are able to make for you, even if it's reduced hours or longer breaks etc.
Don't feel bad your growing a baby they should understand, and you need to rest if to much, say health of you and baby is more important take it easy I didn't and ended up really not well , is there anyway you can go off for a bit then see if you have any annual leave to take before your mat leave