Absence of selection on birth weight in modern Italy.

The effect of stabilizing and directional selection on birth weight has been analyzed for Italian births from 1954 to 1994, a period of rapid improvement in environmental conditions. The population of newborns was subdivided according to gestational age, one of the main covariates of birth weight. In the last cohort of births, no selection at all (neither stabilizing nor directional) was found in full-term babies, which represent more than 95% of total deliveries. Preterm babies are still selected against, even if at lower rates than in the past. It can therefore be claimed that improved and widely available prenatal and neonatal care has dramatically changed the selection patterns previously associated with birth weight in the majority of the Italian population. The mortality rates associated with birth-weight variations lying in a wide interval (2.5 kg-4.5 kg) are nowadays very similar, and both stabilizing and directional selection have practically disappeared.