Such as asking me to reschedule midwife appointments because there isn’t enough staff to cover my absence. when I say no she gets very rude towards me
On this, I'd say try to meet your manager half-way and recognise she is having to balance work between members of her team. If she can't get cover because she has too few people it does give her a problem, so if you can be flexible and are able to rearrange any routine appointments so they align with team coverage I'm sure you'll find her a lot more pleasant. It's no excuse for her to be rude to you, of course, but maybe it explains her behaviour.
She also hasn’t lightened my work load even though a pregnancy risk assessment is in place. And despite my best efforts at trying to explain to her that I’m struggling greatly she just doesn’t seem to care. I don’t think I can cope anymore but I don’t want to cause a massive thing about it
If it's a problem to you, it isn't about 'causing a massive thing' it's about highlighting to your manager that your workload is a problem (which you've already done). Unfortunately just saying that isn't enough, especially if you're already short staffed. The work still has to get done, it won't go away. You mention there is a risk assessment in place, what exactly did the risk assessment say that you may be able to discuss with your manager to back up the fact your current workload is too much? Can you identify any specific tasks that could be reallocated to reduce your workload as your pg advances? Again, I think you need to try to help yourself on this one, suggest a solution not a problem.
It sounds like it's a case of "manage your manager" rather than simply her seeing your pg as an inconvenience.