Recruitment
The initial ALSPAC sample consists of 14,541 pregnancies. This is the number of pregnancies for which the mother enrolled in the ALSPAC study and had either returned at least one questionnaire or attended a "Children in Focus" clinic by 19/07/99. Out of the initial 14,541 pregnancies, all but 69 had known birth outcome. Of these 14,472 pregnancies, 195 were twin, three were triplet and one was a quadruplet pregnancy meaning that there are 14,676 fetuses in the initial ALSPAC sample. Note that of these 14,676 fetuses, 14,062 were live births and 13,988 were alive at 1 year.
When the oldest children were approximately 7 years of age, an attempt was made to bolster the initial sample with eligible cases that failed to join the study originally. As a result, when considering variables collected from the age of seven onwards (and potentially abstracted from obstetric notes) there are data available for more than the 14,541 pregnancies mentioned above. The number of new pregnancies not in the initial sample that are currently represented on the built files is 542. Of these 542 additional pregnancies 6 were twin, meaning that the number of additional children that need to be considered is 548. The total sample size for analyses using child based questionnaire data collected after age seven is therefore 15,224. Note that of the total sample of 15,224 fetuses 14,610 were live births and 14,535 were alive at 1 year.
A 10% sample of the ALSPAC cohort, known as the Children in Focus (CiF) group, attended clinics at the University of Bristol at various time intervals between 4 to 61 months of age. The CiF group were chosen at random from the last 6 months of ALSPAC births (1432 families attended at least one clinic). Excluded were those mothers who had moved out of the area or were lost to follow-up, and those partaking in another study of infant development in Avon.