Safety belts work for everyone, including pregnant women. Like all occupants, they are more likely to be seriously injured if they do not wear safety 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 safety 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 safety belts effective is wearing them properly.
Lap-Shoulder Belt
Safety Belt ExtenderVehicle, Engine and Transmission ID and VIN Location, Derivative and Usage
Note:: Graphic shows exemplary VIN.
The VIN plate is the legal identifier of the vehicle. The VIN plate is
located on the upper left corner of the instrument panel (I/P) and can
be seen through the windshield from the outside of the vehicle:
...
Seat Adjustment
WARNING
You can lose control of the vehicle if you try to adjust a driver seat while
the vehicle is moving.
Adjust the driver seat only when the vehicle is not moving.
To adjust a manual seat:
1. Pull the handle at the front of the seat.
2. Slide the seat to the desired position and release ...
How Does an Airbag Restrain?
In moderate to severe frontal or near frontal collisions, even belted occupants
can contact the steering wheel or the instrument panel. In moderate to severe side
collisions, even belted occupants can contact the inside of the vehicle.
Airbags supplement the protection provided by safety belts ...