Birth weight is considered as a possible crucial intervening variable in a causal sequence that leads from prenatal nutrition to perinatal mortality and retarded child development. Birth weight is shown to have a stronger correlation with perinatal mortality than length of gestation. In New York City, if black infants had the same birth weight distribution as white infants, and the same weight-specific perinatal death rates as they now do, their expected overall perinatal death rate would be the same as for white infants.
Three conclusions are germane to the prevention of low birth weight: first, the fetal growth that leads to important variation in birthweight occurs in the last trimester; second, birth weight is influenced by factors in the wider environment as well as in the maternal environment; and third, birth weight is influenced by factors having their origin and effect during gestation.
The role of maternal nutrition during gestation is then examined as a factor in birth weight. Observational studies of circumstances of wartime deprivation support a nutritional hypothesis, but observational studies of everyday diets in pregnancy, and quasi-experimental studies that supplement nutrition in pregnancy, have given equivocal results.
The hypothesis requires fresh testing. An experimental approach can eliminate or control many extraneous and confounding variables.