There's actually a name for the idea: Chekhov's gun. It's not only about foreshadowing; it's also about not cluttering up a story with elements that won't mean anything. There's a great Wikipedia article about it. He actually said it several different ways, in letters and so forth.
You do see certain conventions of foreshadowing over and over. A woman between the ages of sixteen and fifty is vomiting, or simply spending too long in the bathroom? She must be pregnant.