3.6 Age of Inclusion
Countries that have congenital anomalies surveillance programmes have different criteria for age of inclusion. Some include information about fetuses or neonates with a congenital anomaly ascertained only during the early neonatal period (see Table 3.1), while others include those ascertained up to 1 year of age and beyond (see Fig. 3.10).
The Western Australian Birth Defects Registry includes cases diagnosed up to 6 years of age. Fig. 3.11, derived from Bower et al. (2010) (13), indicates the cumulative percentage of cases with major external and internal congenital anomalies by age at first diagnosis. The authors examined the age at diagnosis for all congenital anomalies reported to the Birth Defects Registry from 2000 to 2001. Nearly 60% of all major congenital anomalies were diagnosed during the first week of life, nearly 70% by the first month, nearly 90% by the first year, and nearly 100% by the sixth year.
Age at diagnosis is a critical component of the case definition. Typically, the higher the cut-off age, the greater the reported frequency of conditions, especially for conditions involving internal organs that might not be evident at birth. For example, whereas external anomalies such as NTDs and gastroschisis are evident at birth, some internal anomalies such as heart defects might not be identified until days, or even weeks or months, after delivery. In addition, certain anomalies may require postnatal confirmation.