The semantic and the phonological representations of a syntactic structure are put in correspondence so that the semantically focused constituent (the new information) contains the phonologically most prominent lexical item. We call this the constraint on focus-to-stress alignment. Different languages comply with this requirement either through syntactic movement to the location of main prominence or through the phonological movement of stress on the semantically focused element. Availability of syntactic movement to achieve focus- to-stress alignment is a consequence of a grammar displaying a positive value for the pro drop parameters. Since infants are very sensitive to prosodic properties since birth, we argue that the location of stress in a sentence can be used by infants to determine the value of the pro drop parameter, even before they utter their first words.