Weight of all births and infant mortality.

Birth weight is the most important determinant of perinatal and infant mortality. The lowest mortality rates in the first week of life are recorded among newborn infants weighing 3500 g or more and the proportion of such infants may be regarded as a measure of optimality of the birth population. There is an inverse relationship between the proportion of heavy newborn infants in a country and its infant mortality rate. In both these respects Iceland, Norway, and Sweden have better experience than England and Wales, Denmark, and the United States of America. The effects of parity, maternal age, social class, and smoking are considered, but it appears that there are still factors that inhibit the intrauterine growth potential of American, British, and Danish fetuses. Elective delivery, use of diuretics, and restriction of diet in pregnancy have shifted the birth distribution to the left and this may have more than counterbalanced the possible beneficial effects. These other factors may adversely affect birthweight distribution in North America and Europe to such an extent as to limit or even damage the favourable position already achieved in health and social development as measured by fetal survival.