Thermal environment and human birth weight

Human birth weight is known to be influenced by several factors, including maternal energy supply, maternal stature, disease status, smoking status and gestation length. This article proposes that the thermal environment may be a further factor influencing birth weight. Experimental animal studies demonstrate clear effects of thermal stress on placental function and birth weight, but may have limited relevance for humans due to between-species differences in pregnancy physiology. Observational studies suggest an inverse relationship between environmental temperature and birth weight within and between human populations. Variation in maternal size, body fatness, pregnancy weight gain and heat production is predicted to influence maternal thermoregulatory capacity, as are the size and composition of the foetus. These associations generate the hypothesis that low birth weight in hot environments may in part represent an adaptation to environmental heat stress.