Source: Office for National Statistics
The age-specific fertility rate measures the annual number of births to women of a specified age or age group per 1,000 women in that age group.
The rates for women under 20 and 40 and over are based on the female population aged 15 to 19 and 40 to 44 respectively.
Age-specific fertility rates for 1981 are based on a 10% sample due to the late submission of some birth registrations.
The population estimates used to calculate fertility rates from 1938 to 1980 are rounded to the nearest hundred and are therefore of a slightly lower level of accuracy than the fertility rates for 1981 to 2014.
The Office for National Statistics (ONS) Birth Statistics Quality and Methodology Information document contains more information on birth statistics.