How long does marijuana stay in a pregnant woman's system?

Depends on the use.: Marijuana is tested in blood, urine, saliva & hair. The cut-off value for detection depends on how much was consumed on a regular basis. Smoking 1 or 2 cigs may cause positive tests for a few days after the last use. More frequent use can cause positive tests for 2 wks. Heavier, daily use may result in positive tests for 12 weeks or longer.