Hello,
Actually the chance of getting an infection from another person who has shingles is very less because it requires direct contact from the shingle lesion to get the virus though there are few incidences of indirect contact transmision. Unlike the chickenpox, the virus in shingles are not found in the respiratory tract therefore airborne transmission is most unlikely.
As your rash is on your back and not exposed to anyone, the chance of transmission is very very less, but of course we cannot definitely exclude the possibility. Therefore the odds of those pregnant women catching the infection is very very low but again there is a very thin thread of possibility.