Pregnant Pause

What is the origin of the phrase 'pregnant pause'?

Metaphor. Some pauses resemble literal pregnancies in that they are filled with meaning, as a pregnant body is filled with the offspring to be born; and they are forward-looking--that is, they have the listener waiting for the next utterance--in something of the same way that a pregnancy portends the forthcoming birth. The Oxford English Dictionary gives examples of the word "pregnant" used with the sense of "full of meaning" beginning in the 15th century.