|Total Prevalence Rate = No. Cases (LB + FD + IA) / No. Births (live and still)
Cases = Cases of congenital anomaly in population
LB = Liveborn
FD = Fetal deaths from 20 weeks gestation
IA = Induced Abortion or termination of pregnancy after prenatal diagnosis, at any gestational age
Birth (live and still) = All live and still births in the population as declared on official birth registrations
Note: Slight discrepancies are present between numerator and denominator, but are not great enough to have an important effect on prevalence rates.
Prevalence Rate at Birth = No. Cases (LB + FD) / No. Births (live and still)
The two measures are equal if there are no terminations of pregnancy after prenatal diagnosis in the population.
Differences in Total Prevalence rates over time or between regions may reflect one or more of the following factors: genetic differences, environmental differences, differences in diagnostic services, differences in the methods of collecting epidemiological data, and even chance differences (see Interpretation of Prevalence Rates).
Differences in Prevalence rates at birth over time or between regions may reflect the same factors as above, but also differences in prenatal screening policies and differences in frequency with which prenatal diagnosis if followed by termination of pregnancy.