Millennium Cohort Study

Unlike the previous three national birth cohorts, which all share the same birth week, MCS cohort members were born over a 17 month period (September 2000-January 2002). This allows researchers to investigate season of birth effects in children’s outcomes and trajectories.

The population from which every child should have a known, non-zero chance of selection was defined as: all those:

• born within eligible dates: between 1/9/2000 and 31/8/2001 (England & Wales), and between 23/11/2000 and 11/01/2002 (Scotland and Northern Ireland),
• alive and living in the UK at age nine months and
• eligible to receive Child Benefit at that age.

The sampling strategy was to make a selection of areas of residence, and within them to recruit 100 per cent of the children born in the eligible period. The statistical geography available for such clusters in 2000-01 was the boundaries of electoral wards as they stood before updating at the 2001 census.

Importantly, certain sub-groups of the population were intentionally over-sampled, namely children living in disadvantaged areas, children of ethnic minority backgrounds and children growing up in the smaller nations of the UK.

By the end of fieldwork for the Age 9-Months Sweep, 18,552 families had been interviewed, and the cohort included 18,818 children, allowing for 246 sets of twins and 10 sets of triplets. This represents a response rate of 72 per cent of all the families with eligible children living at nine months in the sampled wards.

The Age 3 Sweep in 2003-04 provided an opportunity to catch up with families who should have been in the Age 9-Months Sweep, but had been missed because they had only recently moved to an eligible address. This group of ‘new families’ was only recruited in England. The boost to the sample from the ‘new families’ brought the total of all families ever interviewed to 19,243 (and the number of children ever taking part up to 19,517).

There is no sample refreshment by immigrants. All the MCS children were resident in the UK at nine months, and would cease to be eligible if they leave the UK (but resume eligibility on return to the UK).

For further details, please see Plewis (2007).

Objective physical measurements have been taken since the age of 3. They include height, weight, waist measurement and body fat.  At ages 7 and 14, activity monitors were worn in order to measure physical activity and sedentary behaviour.

At age 14, cohort members and their residential natural parents were asked to provide a saliva sample for DNA extraction and subsequent genotyping. This is the first time a triad of DNA samples has been collected from 2 biological parents and their child in a large scale nationally representative study.

Administrative data from relevant government departments and agencies have been linked to the survey data in the domains of health and education. The data are available to access through the UK Data Service. Up to age 16, these linkages are based on parental consent collected at various time points. At age 17, we asked cohort members for their own consent.

Health
Linked health data are available from birth registration records and maternity records based on parental consent shortly after birth. We are working to link Hospital Episode Statistics (HES) for England and have already linked some health data for Scotland and Wales.

Linked health data are available from Scottish Medical Records (SMR) for 2000-2015 (outpatient, inpatient and day care, maternity, Prescribing Information System).

For Wales the number of inpatient and day case hospitalisations at ages 0-11 are available. A range of Welsh health records up to cohort members’ 14th birthdays can also be linked to the cohort data through the SAIL Databank, including Child Health, Emergency Department, Patient Episode Database and Primary Care GP data.

Education
For cohort members in England, pupil-level linked education data from the National Pupil Database are available for Key Stages 1, 2 and 4, including exam results as well as information on absences, ethnicity, languages spoken at home, special educational needs, and eligibility for free school meals. School-level data are also available.

For Scotland and Wales, Key Stage 1 data are available through UKDS. For Wales, education records up to and including Key Stage 4 can also be linked to cohort data through the SAIL Databank.

Future data linkage
At age 17, cohort members were asked for their permission to link data from an extensive range of administrative records to the MCS survey data. These include information on:

• health (from hospital records)
• education and training records after age 16
• higher education applications and offers
• payments of student loans
• records kept by the Department for Work and Pensions and HM Revenue & Customs, including about benefit claims, participation in employment programmes, jobs, earnings, tax and National Insurance
• crime records.

We are pursuing these linkages with the aim of making the linked administrative data, including data currently only accessible through the SAIL Databank, available via the UK Data Service. We are currently applying to link to HES data which will include COVID-19 data and plan to refresh the SMR linkages.