All pregnant women living in the geographical area of Avon, UK, who were expected to deliver their baby between 1st April 1991 and 31st December 1992, were recruited. 14,541 women were enrolled. Amongst these pregnancies, there were a total of 14,676 fetuses, resulting in 14,062 live births and 13,798 children who were alive at 1 year of age and were singletons.