LGoR - Its my first pregancy, expecting non-id girs, and I'm finishing work this week at 28 weeks. My midwife advised me not to work past 28 weeks as at this stage both babies can be 3lbs i.e. you can be as big as a singleton pregnancy at full term. I'm really pleased that she said that because I've always planned to stop now, and it's really not a moment too soon for me. I'm very tired and uncomfortable and not sleeping very well so finding working a bit of a trial. I have also been amazed at how quickly I've got bigger - at 20 weeks just had small bump, now at 27 struggle to turn over in bed at night/get up off the sofa. I've put on 13lbs so far, so it's not excessive.
Obviously it depends what sort of work you do and how far you travel etc. I commute to London 1.5 hours each way door to door, usually walk to station but now driving (finding that quite difficult now too) and sitting at desk/in meetings all day so uncomfortable causing back pain etc. A colleague who is also expecting twins planned to work until at least 33 weeks and had to stop at 30.
It's been quite easy for me to plan this way because I don't have anything to compare with, so I didn't have any expectations of myself working later, but I would suggest at least giving yourself the option of stopping earlier if you feel you need to.