All pregnant women residents in that part of the old administrative county of Avon comprising the three Health Districts shown in Figure 2 were eligible to participate in ALSPAC if their estimated delivery date (EDD) fell between 1 April 1991 and 31 December 1992 inclusive. Any resulting child from these pregnancies is considered eligible. The catchment area covered the three health administration districts within the South-West Regional Health Authority that became the ‘Bristol & District Health Authority’. This area (1991 total population ∼0.9 million) includes the City of Bristol (1991 population ∼0.5 million) and surrounding urban and rural areas, including towns, villages and farming communities; but excludes the area of Avon around the City of Bath. Pregnant women migrating into the catchment area were eligible up to the point of delivery; pregnant women originally resident in Avon but migrating out of the catchment area prior to delivery were excluded unless they had completed the questionnaire scheduled for the third trimester of pregnancy. A small number of women and their children falling outside the above strict definition of eligibility contributed data and have been included in ALSPAC. This is discussed below.
