When to take off for maternity leave?

OK my manager told me to ask my doctor so i did. My doctor told me i can work up till my due date!! Which has to be wrong.. I work at fort knox and in my job i do alot of bending over, reaching, stretching, twisting and standing on my feet for 6-8 hours straight. If i keep it up i will go into preterm labor.. I just hit my 7months due in JUNE.. Someone please help me out!!