The Collaborative International Dictionary of English v.0.48:

birthrate \birthrate\n.
   the ratio of live births in an area to the population of that
   area; usually expressed as birthes per 1000 population per
   year.

   Syn: fertility, fertility rate.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

birthrate
    n 1: the ratio of live births in an area to the population of
         that area; expressed per 1000 population per year [syn:
         birthrate, birth rate, fertility, fertility rate,
         natality]