What causes tomatoes to fall prematurely? Stum607

I have recently had a problem of unripe tomatoes falling from plants. These unrip fruits are about half the normal size. What is the probable cause of this condition? Stum607

Someone told me this is because they are lacking calcium and told me to crush egg shells and put them on top of the soil. I'm waiting to hear back from the nursery this morning before trying that - I'm having the same problem.