In the case of survey data, the adolescent birth rate is generally computed
based on retrospective birth histories. The numerator refers to births to women
that were 15 to 19 years of age at the time of the birth during a reference
period before the interview, and the denominator to person-years lived between
the ages of 15 and 19 by the interviewed women during the same reference
period. Whenever possible, the reference period corresponds to the five years
preceding the survey. The reported observation year corresponds to the middle
of the reference period. For some surveys, no retrospective birth histories are
available and the estimate is based on the date of last birth or the number of
births in the 12 months preceding the survey.