Oh dear... Nice to see Ann Oakley's legacy is alive and well in Mumsnet 'Pregnancy is a condition not a disease'
Yes us women were designed to be pregnant and give birth but not do that and take on the whole bloody world at the same time.
Want my advice - I think you are coping marvelously - I think you are doing your job and someone elses... and pregnant or not you need to learn to say 'NO'
Look... its very simple to say - 'Go off sick' or 'Start ML' but it's obvious that your job has a level of responsibility over and above other peoples so you need to start prioritising. You need to say to yourself first and foremost - I have to do this list, I can possibly do that list and that other list is not my problem.
Then you take your lists to your head and tell him where he needs to find cover or you will have to start your ML early - ie there is a choice - an acceptance that you will do a certain amount of work and handover the rest or no work as you will be forced to stop working.
Now there are ways to do this without pissing off your manager - I wont bore you with the details as I think you come across as very capable in the work-place.
Women, especially those in jobs with high levels of responsibility can easily take on much more than their male equivalent and when pregnant they seem to go into hyper-drive as some sort of apology... I know because I have been there.
If you continue to take all this work on then they will never support you... its the way of the world unfortunately.
My background is midwifery - I have felt the pressure you are talking about to stay until the bitter end. I have done the shifts where I havent been able to leave a ward area for a wee for 9 hrs, not eaten or drank, etc and I ended up threatened prem lab - I went back to work (against all advice) but I made it my mission to ensure that I have 2-3 wee-breaks a shift, I took low-risk ladies and I ALWAYS ate and drank. Work wasn't suddenly less busy and sure my colleagues did more work but as I was still working my arse off until 37 weeks at a time when we were very short staffed they were just grateful I was there working.
In short - no one can or will do it for you.
As a parent that is on the quick-dial of the school's senco - personally if I was shoved down a list because of a SENCO going off pregnant then I would kick the school and LEA's arse because it's not like it snuck up on them like sickness.
I don't need to tell you that the LEA have responsibilities towards statementing processes, LAC reviews and as such they are your safety net - you a vehicle of a bigger organisation... you there or not the LEA have to ensure your job is done and if it's not then THAT is NOT your responsibility.
I know there are parents who would take it out on you but it sounds like you have that risk anyway....
My son was badly failed by a school but there were a number of professionals culpable in that - not one single person...
Look after yourself - you seem dedicated and passionate - give yourself a break.