Hey, M.,
Congrats! Twins are the best. But the pregnancy really is different with two than it is with one (I noticed that most of the people who have responded so far did not carry twins). It's really important to keep in touch with your doctor about your work. It may be that, because of your commute, your doctor will feel you need to stop earlier. I started having blood pressure issues at around 31 weeks, and my doctor ordered me not to take the stairs at work, not to take the subway and do a lot of walking around the city, etc. (I drive to work on Long Island, so that wasn't a real problem). At 32 weeks, my blood pressure was so high at a routine appointment that my doctor told me to stop working immediately. Because she "disabled" me, I got short-term disability. She didn't put me on bed rest -- just wanted me to take it easy. I delivered my girls at 36 weeks.
Obviously, every pregnancy is different. But every one I know who has twins gave birth around 35-36 weeks and had stopped working at least a month earlier.
Feel free to message me privately if you have any questions. Good luck with the rest of the pregnancy!!
-A.