When should I stop working while pregnant?

Can work until end. Unless you develop a condition such as high blood pressure or preterm labor, most woman can work until the end of their pregnancy. Some woman have jobs which require heavy lifting or other physical activities and may need to modify their job duties towards the end. Usually your doctor will let you know if you need to stop working, but you can always ask if you are concerned about something.
Varies with case. I have known many women , mainly nurses, who went into labor at work (at term), and some that were put on total bed rest from 22 weeks.Each case must be handled as unique by those who know you, your situation & the type of work you do.