http://www.ncbi.nlm.nih.gov/pmc/articles/PMC27812/pdf/908.pdf

\"To examine trends in mortality among babies registered solely by their mother (lone mothers) and to compare these with trends in infant mortality for couple registrations overall and couple registrations subdivided by social class of fatherInfant mortality has long been accepted as an important indicator of a population's health, with evidence of any social differentials in this indicator regarded as particularly unaccetable and a spur to action. The traditional way of analysing social trends in infant mortality, often limited to births inside marriage by social class of father; has, however, become increasingly problematic as growing numbers of infats are excluded from such an analysis, not least the babies of lone mothersOver the past 20 years there has been a clear reduction in the proportion of births registered within marriage in England and Wales, and this trend has been accentuated in manual social classes.\" (The authors)