URGENT - Job stress while pregnant

[minicorrect]

I'm very lucky to be a PA who's had some amazing career progression opportunities since I returned from maternity leave in Jan last year, however, I'm now pregnant again and really struggling with all the responsibility.
I currently support the Chief Executive, Operations Director and Finance Director as well as any global leadership team members that visit the office (there's usually at least one a week) as well as having a regional lead role (and commuting 2 hours each way to work). However, I took the regional lead role before the CE started unaware I was going to be expected to support him as well (which is essentially a full time role in itself), and this has now had to take a back seat while I try to juggle everything else, despite it being the part of my job I actually enjoy.
While all my bosses have been very supportive of my pregnancy, my line manager is very blase about it all and took very little interest in the risk assessment and doesn't seem to be taking my requests for help seriously even though I've identified a person who could share some of my workload when they return from ML in April.
DP thinks I should just call in sick with stress, but I think I should speak to HR first to raise the issues even though he's very pally with my bosses and will go straight back to them.
Can anyone offer any advice on how to approach this without making things worse for myself? My DD was born underweight and the consultant is being cautious this time to try and avoid this happening again so I wonder if I actually have medical grounds for requesting a lower stress role for the next few months, knowing there's a link to low birth weight and maternal stress.
Understandably, this is also affecting my relationship with DP and we spent most of my week off arguing which hasn't helped either. I just feel at my wits end with nowhere to turn.
Sorry for the long me post!!

Thanks MadamDS. I agree that getting my boss to take the risk assessment more seriously would be a good start, but it was a blank template and he just told me to put whatever I wanted in it and he'd sign it off - without even looking at it.
The proposal is that the woman returning from ML supports the CE and another team while I continue to support the OD and FD and then we get someone in to cover my ML from June/July. But he doesn't seem keen for her to do this and has suggested we get someone in for my cover sooner on the assumption we will find someone willing to do a role this demanding. And even if we do we're looking at a couple of months to get the recruitment process sorted/notice given, etc. Which doesn't really help me now!
I tried the angle that this woman is coming back and has to be paid anyway (and is on the highest salary in the admin team) so it makes sense to give her a role that reflects this. It seems like a no brainer but he is putting barriers up and I don't understand why. One comment he did make was that he is going to struggle to find someone trustworthy which is a compliment but doesn't really help!

I really get the importance of having someone they trust. I wouldn't put anyone in that sort of role without absolute trust - there may be issues with your proposed candidates that you don't know.

On the other side of your post, my understanding is that they have to do a risk assessment. That must include stress. They then need to look to minimising the risks as much as poss. And if the job would still be too risky they have to put you on something else or send you off on paid leave. (This is all from memory. Do wait for flowery or someone else who knows their stuff more confidently - but I think you can make some more headway with this).