Maximum permissible radiation dose in pregnancy is –
Maximum permissible dose in pregnancy is 5 mSv (0.5 rem). 

Rem = Rad x Quality factor.

Main radiation exposure during pregnancy can be by X-rays (radiography or CT), and the quality factor of X-rays is 1. 
That means Rem = Rad.
Therefore, the maximum permissible dose in pregnancy is 0.5 rem or 0.5 rad.
0.5 rad
1.0 rad
1.5 rad
2 rad