What happens in a hit and run accident?

Well, for you it's all no problem. You were rear-ended, so it's totally not your fault no matter what. And since your vehicle didn't really sustain any damage (get it looked at anyway), it's absolutely not a problem.

The pregnant girl who was pushed into you might have a problem, especially if a) she didn't have Collision coverage and b) nobody can identify the offending vehicle that took off, but that's not your problem. Because she may still be on the hook for the damage to your vehicle (nobody said this stuff was fair), and only Collision coverage responds to hit-and-run damage.

As for the douche bag who caused the whole thing and ran away like a little b*tch, he'll probably get away with it. Maybe he'll be caught, because cops do actively look for hit & run offenders who cause injuries, but probably not.

You can file for medical payments from your insurance if necessary. The police should be loking for the drive-away.

It;s unfortunate that she had to go to the hospital but her insurance will cover her care and the repair of the car.

If the police ever locate the person who caused the accident the insurance company will sue them in court to get their money back.

Unfortunately, when somebody commits a crime and gets away, nothing happens to them. If there were witnesses to the accident that got a vehicle description or a license plate number, the person will be found, held responsible, and criminal charges would be filed. Sadly, there is little chance of this happening. Even if the police get a vehicle description or partial license plate number, this kind of thing is pretty low on their list of priorities, and they often don't give it 100% effort.