PubMed · 7214871
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A F Lewis. 1981. Linking birthweight and birth registration data: a postscript.. https://pubmed.ncbi.nlm.nih.gov/7214871/
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Data from the Medical Birth Registry of Norway were used to estimate sibship correlations in large sibships (each with > or = 5 infants among singleton live births surviving the first year of life), while adjusting for covariates such as infant gender, gestational age, maternal age, parity, and time since last pregnancy. This sample of 12,356 full sibs in 2,462 sibships born in Norway between 1968 and 1989 was selected to maximize the information on parity, and a robust approach to estimating both regression coefficients and the sibship correlation using generalized estimating equations (GEE) was employed. In concordance with previous studies, these data showed a high overall correlation in birth weight among full sibs (0.48 +/- 0.01), but this sibship correlation was influenced by parity. In particular, the correlation between the firstborn infant and a subsequent infant was slightly lower than between two subsequent sibs (0.44 +/- 0.01 vs. 0.50 +/- 0.01, respectively). The effect of time between pregnancies was statistically significant, but its predicted impact was modest over the period in which most of these large families were completed. While these data cannot discriminate whether factors influencing birth weight are maternal or fetal in nature, this analysis does illustrate how robust statistical models can be used to estimate sibship correlations while adjusting for covariates in family studies.
This review discusses the use of Nordic Medical Birth Registers (MBRs) in epidemiological studies, paying special attention to the topics that have been investigated and to other data sources that increase the usefulness of MBRs. We divided the reviewed studies into four groups according to the data sources on which they were based: (1) studies using MBR data only; (2) studies combining maternal or paternal background information, obtained from other data sources, with MBR data; (3) studies combining MBR data with subsequent outcome information on newborns; and (4) studies using information about consecutive pregnancies and generations. Our review shows that MBRs are good sources for studying the following topics: maternal biological and obstetric background; interventions and health care during pregnancy and birth; newborns' short-term outcome; and the relations between these factors. In addition, the usefulness of MBRs increases when the data they contain are combined with data from other sources. We found that data from more than twenty different sources have been linked with MBR data in the Nordic counties. As time passes, national MBRs become a useful source of information for studies on consecutive pregnancies or generations. In addition, the associations between pregnancy, delivery, perinatal health and long-term outcome can be studied by using an MBR as the basic data source.
Misclassification frequently leads to bias in epidemiological studies, and causes concern for perinatal epidemiologists interested in using birth certificates as a data source. We used a maximum likelihood method to estimate the classification probabilities (conditional probabilities that indicate the probability of classification in a particular category, given the person's true category) of two data sources for a three-category outcome of prenatal care. The probability that women receiving adequate or inadequate care were correctly classified was estimated to be greater than 90%. The probability was much lower (< 35%) that women receiving intermediate care were correctly classified. The misclassification women from the intermediate category resulted in poor predictive values (< 70%) of women classified as receiving either adequate or inadequate care. Because of these findings, we combined the adequate and intermediate categories to form a two-category classification system. This revision resulted in higher positive predictive values (> 90%) with only a slightly lower classification probability (> 85%) for the combined category. We conclude that the degree of accuracy for a two-category classification of prenatal care based upon birth certificate information is acceptable, but we question the accuracy of indices of prenatal care with more than two categories.