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. 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
The following linked health data are available to access through the UK Data Service:

• Linked health data from birth registration records and maternity records based on parental consent shortly after birth.
• Linked Hospital Episode Statistics (HES) for England up to 2020. We are in the process of updating the data to 2023, including COVID-related ICD-10 codes Scottish Medical Records (SMR) for 2000-2015 (outpatient, inpatient and day care, maternity, Prescribing Information System).
• Welsh health records up to 2020-2021 for cohort members and parents (deaths, emergency, outpatient and hospital inpatient records, general practice data). Historic and up-to-date Welsh health records can also be linked to the cohort data through the SAIL Databank.

A range of up-to-date linked health data (such as HES, mental health, cancer) for England and Wales are also available to access through the UK Longitudinal Linkage Collaboration (UK LLC).

Education
The following linked education data are available to access through the UK Data Service:

• Pupil-level linked education data from the National Pupil Database (NPD) in England for Key Stages 1, 2, 4 and 5 ). These include 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. Plans are also underway to link NPD data via the UK LLC.
• For Wales, education records up to and including Key Stage 4. These, as well as later education records, can also be linked to cohort data through the SAIL Databank.
• Some Scotland, Key Stage 1 data, with linkages in progress for more Scottish education school records (including qualifications, attendance and leavers data).

Student Loans Company (SLC) information will soon be linked. This will include information on loan/grant entitlement, customer payments and institution attended.

Economic
The UK LLC will soon be linking 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.