Cohort profile

A number of ALSPAC cohort profile papers are available describing the generations. Short summaries of recruitment and representativeness are also available below. 


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 recorded from this moment on there are data available for more than the 14,541 pregnancies mentioned above. The number of pregnancies not in the core sample that are currently represented on the built files and reflecting enrolment status as of February 2019 is 913 (456, 262 and 195 recruited during Phases II, III and IV respectively). The total sample size for analyses using child based data collected after the age of 7 is therefore 15,454 pregnancies resulting in 15,589 known foetuses. 14,901 were alive at 1 year of age.

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.


A number of studies have been undertaken to assess the representative nature of the ALSPAC sample that was completing questionnaires or attending the Children in Focus clinic with (a) the total Avon area population, and (b) the whole of Great Britain, in 1991.

The population of parents and children living in the study area in 1970 were broadly similar to those of the rest of Great Britain. The 1991 census was used to compare the population of mothers with infants under 1 year of age resident in Avon with those in the whole of Britain. The results are shown below:

  AvonWhole of Great Britain
Owner Occupier 68.7% 63.4%
1 + person/room 26.0% 30.8%
Car in household 83.7% 75.6%
Married couple 71.7% 71.8%
Non-white mother 4.1% 7.6%

Thus the mothers of infants in Avon were slightly more likely than those in the rest of Britain to live in owner occupied accommodation, to have a car available to the household and to be less likely to have one or more persons per room and be non-white.

The comparison of the ALSPAC population completing questionnaires 8 months post-delivery with the whole eligible Avon population is shown below:

Owner Occupier 79.1% 68.7%
1 + person/room 33.5% 26.0%
Car in household 90.8% 83.7%
Married couple 79.4% 71.7%
Non-white mother 2.2% 4.1%%

Thus, similar to all studies where a representative sample has been attempted, this study had a slight shortfall in the less affluent families (those living in rented accommodation, not having a car or being single or unmarried cohabiting). The study had a shortfall in ethnic minority mothers.

A comparison of the growth standards developed from the ALSPAC Children in Focus group with published national standards is shown below. Weights and birth lengths are in remarkable accord, and growth measures are very similar:

Measurements at birth, 1 and 2 year clinics: ALSPAC cohort vs. UK 1990 data

  Birth1 year clinic (mean age 1.03 yrs)2 year clinic (mean age 2.08 yrs)
weight (kg)mean 3.55 3.55 10.54 10.15 13.03 12.53
sd range 3.05-4.05 3.08-4.05 9.38-11.70 9.16-11.24 11.57-14.47 11.29-13.92
Length (cm)mean 51.26 51.09 76.53 76.23 87.54 87.82
sd range 49.31-53.21 49.07-53.11 74.08-78.98 73.66-78.75 84.60-90.48 84.57-91.04
weight (kg)mean 3.42 3.41 9.84 9.73 12.42 12.29
sd range 2.99-3.87 2.96-3.87 8.83-10.85 8.79-10.77 11.11-13.67 11.06-13.70
Length (cm)mean 50.41 50.21 74.60 74.43 86.17 86.49
sd range 48.60-52.22 48.33-52.09 72.34-76.86 71.97-76.83 83.26-89.08 83.36-89.58
Edit this page