Out of financial necessity or out of choice for the stimulating satisfaction that working gives, pregnant women have to or choose to work through their pregnancies.
And if they do so how long into their pregnancy can and should they work is a question asked here.
For the answer to this question, women who have no medical reason not to work can continue to work right until the onset of labor. 
As long as the pregnancy is normal and complication free and the job poses no hazard of any sort (the presence of chemicals and radiation could pose hazards) a woman can continue to work until such time as she is comfortable doing so and remains in good health.
If a woman’s job requires her to stand for long periods of time (say a hair dresser) then the last trimester would be difficult for her and a doctor may advise rest in the final few months.
If a pregnancy is high risk, then a woman may be advised only light activity throughout the pregnancy. So with these exceptions, there is no reason not to keep working though a pregnancy or until a woman is physically able and comfortable in her place of work and given job. Being active and alert throughout the pregnancy can only be good for the ultimate delivery.
