No correlation in post-neonatal deaths for twins. A study of the early mortality of twins based on the Norwegian Medical Birth Registry.

The records of 10,204 pairs of twins born in Norway 1967-84 in the Norwegian Birth Registry were examined. The probabilities of stillbirth, neonatal death and post-neonatal death, conditional on the fate of the cotwin, were calculated. The neonatal death rate was four times increased when the cotwin was stillborn. When the cotwin died neonatally, the probability of postneonatal death was increased ninefold. The concordance rate for stillbirth and for neonatal death was above 0.4. In striking contrast, there was no correlation in postneonatal death for members of a twin pair. This finding suggests that genetic or common environmental factors play a minor role in the aetiology of post-neonatal death.