a1 Department of Informatics, University of Oslo, Norway
Using medical records from maternity clinics in the two Norwegian cities Oslo and Bergen, the effect of lactation on infant mortality during the period 1860–1930 is examined, comparing those who were and were not breastfed in a total of about 6900 live born infants. The mortality of children not breast-fed was nearly three times that of those who were breast fed. In a Cox regression analysis the infant's year of birth and the mother's marital status were found to influence mortality in addition to lactation. Children born to unmarried mothers experienced a mortality about twice that of those born to married mothers, both during on-going lactation and in the absence of lactation, up to about 1915. Children born in Oslo had a slightly higher mortality than those born in Bergen.
The duration of lactation was found to have a continuing protective effect on infant survival after weaning—the longer the duration, the lower the mortality after cessation of lactation.
(Received July 01 1988)