Age specific fertility rate (ASFR) is more refined and specific fertility measure than general fertility rate (GFR) because age specific fertility rate (ASFR) considers the exact age of women while calculating the fertility rate. General fertility rate tells us about how many infants are born in a specific year by the female of reproductive age whereas ASFR finds out the number of births performed by the women of a specific age group. We can clearly know the fertility rate of the women of a certain age grouped with the calculation of ASFR. The reproductive capacity of the women of different age groups is not the same. So, we can say age-specific fertility rate (ASFR) measures the annual number of live births to women of a particular or specified age or age group per thousands women in that particular or specified age group. An age-specific fertility rate (ASFR) is normally computed as a ratio. Age specific fertility rate (ASFR) is expressed as births per thousand (1,000) women.

For example, women produce more children at the age of 20-24 and 25-29 because their reproductive capacity is high at these groups. After that, the fertility rate keeps on decreasing.

Therefore, age specific fertility rate is the total number of births given by the women of a specific age (or age group) per thousand of the same age in given year.

For convenience, one can divide the women of reproductive age in 5 years age interval and ASFR is calculated for each age grouped separately. In this way, fertility rate for each age group is separately calculated. For this, we require the data on the number of births by the mothers of certain age and the number of mothers of that age group ASFR is calculated by dividing the total number of live births given by the women of a specific age with the total number of that age grouped and multiplying the product by 1000.

The following formula is used to calculate ASFR:

ASFR_{a }= B_{a}/Pf_{a}*1000

Where,

B_{a }= the total number of live births given by the women of age ‘a’ in a place in a given year.

Pf_{a} = the total number of mid-year female population of age ‘a’ in the same year.

A = age-group of women of reproductive age, i.e., 15-19, 20-24, 25-29, 30-34, 35-39, 40-44, and 45-49

ASFR_{a} = Age specific fertility rate for age ‘a’

**For example**

Suppose, the total population of females of age group 20-24 was 35000 in 2007 in Birgunj. The women of the same age group gave birth to 2500 babies in the same year. Calculate age specific fertility rate (ASFR).

Solution

The total number of live births given by the women of age group 20-24 (B20-24) = 2500

The total midyear population of women of the same age group (Pf20-24) = 35000

Age specific fertility rate for the age group 20-24 (ASFR_{20-24}) = ?

Here,

ASFR_{20-24} = B_{a}/Pf_{a }* 1000

= 2500/35000 * 1000

= 71.42

= 71.42 infants per 1000 women

Therefore, in Birgunj, per thousand women of age group 20-24 gave birth to about 71 babies in the year 2007.