Pregnant Pause

A pause that gives the impression that it will be followed by something significant (Source: http://en.wiktionary.org/wiki/pregnant_pause)

A pause which those listening expect to be broken by some significant sound or speech i.e. a pause which is expected to "give birth" to something significant. (Source: http://www.answerbag.com/q_view/467796)

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.
(Source: http://www.phrases.org.uk/bulletin_board/7/messages/349.html)

Pregnant pause in Comic Timing:
A pregnant pause (as in the classical definition, "many possibilities") is a technique of comic timing used to accentuate a comedy element, where the comic pauses at the end of a phrase to build up suspense. It's often used at the end of a comically awkward statement or in the silence after a seemingly non-comic phrase to build up a comeback. Refined and perfected by Jack Benny, the pregnant pause has become a staple of stand-up comedy.
(Source: http://en.wikipedia.org/wiki/Comic_timing#Pregnant_pause)