| Variable | Sweep of data collection* | Units of analysis |
| Individual | ||
| Birthweight | First | z-score adjusted for gestational age and gender |
| Child’s gender | First | Male or female |
| Child’s ethnicity | First | White; mixed; Indian; Pakistani; Bangladeshi; black; other |
| Breastfeeding duration | First | Never breastfed; breastfed for <17.4 weeks; breastfed for ⩾17.4 weeks |
| Introduction of solid foods | First | Weeks: <17.4 or ⩾17.4 |
| Television viewing | Second | Daily hours: <1; ⩾1 and <3; ⩾3 |
| Family | ||
| Maternal socioeconomic circumstances | First | Managerial and professional; intermediate; small employers and own account; lower supervisory and technical; semi-routine and routine; never worked and long-term unemployed |
| Maternal highest academic qualification | First | Degree (highest); diploma in higher education; A/AS/S levels; O level/GCSE grades A–C; GCSE grades D–G; other; none (lowest) |
| Lone motherhood status | First | Non-lone mother or lone mother |
| Age at first live birth | First | Years: 14–19; 20–24; 25–29; 30–34; 35+ |
| Age at MCS birth | First | Years: 14–19; 20–24; 25–29; 30–34; 35+ |
| Parity | First | Cohort child first live born (nulliparous) or not first live born (multiparous) |
| Number of children in the household (including cohort child) | Second | 1; 2 or 3; 4 or more |
| Household income | Second† | Per annum: £0–11 000; £11 000–22 000; £22 000–33 000; £33 000+ |
| Maternal smoking during pregnancy (at 3 months’ gestation) | First | Daily: none, 1–9, 10–19, 20+ |
| Parental overweight when cohort child was aged 9 months | First | BMI (kg/m2): Both parents <25; mother only ⩾25; father only ⩾25; both parents ⩾25 |
| Maternal prepregnancy overweight | First | Yes (BMI ⩾25 kg/m2) or no (BMI <25 kg/m2) |
| Maternal employment‡ | First and second | Hours worked per week: never worked; 1–20; 21+ |
| Whether child has meals at regular times | Second | Always, usually, never or sometimes |
| Community | ||
| Easy access to food shops and supermarkets | First | How common: very, fairly, not very, not at all |
| Neighbourhood conditions: noisy neighbours, rubbish, vandalism, pollution§ | First | How common: very, fairly, not very, not at all |
| Satisfaction with area where family lives | First | Very satisfied, fairly satisfied, neither satisfied nor dissatisfied, fairly dissatisfied, very dissatisfied |
| Whether there are any places where children can play safely | First | Yes or no |
| Access to a garden | First | Yes or no |
| Area | ||
| Country | First | England, Wales, Scotland, Northern Ireland |
| Ward type | First | Advantaged, disadvantaged, ethnic |
*Children were aged approximately 9 months at the first contact and 3 years at the second contact.
†Values from the second contact were used unless missing; values from the first were substituted.
‡Average hours worked per week during the weeks worked from birth to the second contact.
§A composite of neighbourhood conditions based on how common the following were: noisy neighbours or loud parties; rubbish or litter lying around; vandalism and deliberate damage to property; pollution, grime or other environmental problems.