Seat belts work for everyone, including pregnant women. Like all occupants, they are more likely to be seriously injured if they do not wear seat belts.

A pregnant woman should wear a lap-shoulder belt, and the lap portion should be worn as low as possible, below the rounding, throughout the pregnancy.
The best way to protect the fetus is to protect the mother. When a seat belt is worn properly, it is more likely that the fetus will not be hurt in a crash. For pregnant women, as for anyone, the key to making seat belts effective is wearing them properly.
Lap-Shoulder Belt
Seat Belt Extender#13-00-89-004D: Car and Truck Fix it Right the First Time Issues - (Nov 19, 2013)
Subject:Car and Truck Fix it Right the First Time IssuesModels: 2014 and Prior GM Cars and Trucks This bulletin has been revised to include updated information. Please discard CorporateBulletin Number 13-00-89-004C.In order to access this bulletin electronically, go to the SI Home Page and selec ...