In general, fertility rates based on the year of registration of a birth are relatively similar to those based on the births experienced within a particular year. However, this may not be true where there are significant lags between the occurrence and registration of births. Although there is a legislative requirement to register a birth within 60 days, the interval between the occurrence and registration of a birth often exceeds this period. The delayed registration of large numbers of births may result in the underestimation of fertility rates during the years in which their registration is delayed, and the overestimation of fertility rates during the years in which these births are subsequently registered.
Where there are significant delays in the registration of a relatively large number of births, fertility rates based on year of registration data may not accurately reflect the level of fertility in any given year and should be interpreted with caution. When delays in birth registrations occur in large jurisdictions the fertility rate for Australia may also be affected.