The relation of measured intelligence to birth weight and duration of gestation.

There have been two main approaohes to investigation of the relation of birth weight to intelligence. In one, birth weights of children of low intelligence are compared with those of children of average or high intelligence. Where the comparison has been between children in ordinary schools, as in Ascher & Roberts's study (1949) in primary, grammar and secondary schools, birth weight differenoes have usually been unimpressive or absent; but among the subnormal, mean birth weight appears to be reduced and the reduction is present even after exolusion of those whose low intelligenoe is associated with physioal abnormalities (Barker, 1966). The more usual approaoh has been to measure the intelligence of children of different birth weights. Numbers investigated by this prooedure are, as a rule, rather small and the methods of analysis sometimes make it diffioult to assess the results. However, the general conolusion which has been reached is that when ohildren with physioal abnormalities-suoh as blindness, deafness and oerebral palsy-are exoluded, the intelligence of the remaining ohildren of low birth weight is about normal (McDonald, 1964). In ohildren of very low weights, however, intelligence appears to be reduoed (Drillien, 1964). The relation of intelligence to duration of gestation is even less well established, but the contemporary viewpoint is probably summarized in Baird's conclusion (1959) that 'there is no clear indication that within wide limits premature expulsion from the uterus does the foetus any serious harm ~. The data used in the present investigation were described in a preceding paper (Record, MoKeown & Edwards, 1969). Briefly, they are derived from observations on all (86,630) Birmingham live births in the period 1 January 1950 to 1 September 1954. Verbal reasoning scores from the results of the eleven-plus examinations were matched for 50,172 children and birth weight and duration of gestation (estimated to the nearest week from the first day of the last menstrual period) were available from obstetrio reoords for 41,534 single births.