Over the past few years car manufacturers have stopped fitting lap belts for the middle rear passenger. All new cars now are fitted with three point seatbelts for all passengers. That is because the injuries which can be caused by a lap belt are extremely serious and potentially life threatening.
When you are involved in a road traffic accident a seatbelt basically serves to slow your body down and stop it travelling forward when the car has already slowed and stopped. Basically, your body continues to move forward at the speed at which the car was travelling until it is restrained and stopped. Three point seatbelts especially those with pre-tensioners are designed to do this by applying as little force as possible to your body.
With the old fashioned lap seatbelts the entire force was applied across your stomach, which could cause severe damage to your internal organs. These are injuries which could prove fatal.
However, even three point seatbelts are not one hundred percent safe. Where the forces involved in a road traffic accident are sufficiently serious the seatbelt can cause injuries and fractures to your sternum (chest plate) and ribs.
If the forces are particularly significant damage can still be caused to internal organs.