Defining Smallness for Gestational Age in the Early Years of the Danish Medical Birth Registry

BACKGROUND Being born small for gestational age (SGA) is associated with decreased insulin sensitivity and increased blood pressure in childhood, but the association with clinical disease in early adulthood is less certain. The Danish Medical Birth Registry has registered all births in Denmark since 1973, but due to variable data quality, data is most often used only from 1981 onwards, and birth registers in other countries may have similar problems for the early years. We wanted to examine whether the data can be used for identification of children born SGA and used in future research. METHODOLOGY/PRINCIPAL FINDINGS All persons born between 1974 and 1996 were identified in the Danish Medical Birth Registry (n = 1.704.890). Immigrants and children without data on gestational age and birth weight were excluded, and a total of 1.348.106 children were included in the analysis. The difference between the different variables used in the history of the registry were examined, and the quality of data in the birth registry from 1974-1981 was examined and compared to subsequent years. Data on birth weight and gestational age in the early years of the registry is inconsistent, and the identification of children born SGA is inaccurate, with 49% false-positives. The biggest source of error is due to the rough and inaccurate intervals used for gestational age. By using -3 standard deviations as a cut-off for the identification of children born SGA, the number of false-positives was reduced to 9%, while the amount of false-negatives were increased. CONCLUSION Choosing -3 standard deviations for identifying children born SGA is a viable, though not optimal solution for identifying children born SGA. Overall the data in the registry is of sufficient quality to be used in further medical research.