Table 1 Assessment of socio-demographic confounders

Age

Participants reported their age when they were approached to participate in the study in early pregnancy.

Ethnicity

Participants reported their main ethnicity (European, Indian, Pakistani, Bangladeshi, Afro Caribbean, African, Middle Eastern, Far East Asian, South East Asian or Other) which was classified as White, Black, Asian or Other.

Relationship status

Relationship status was indicated by whether or not participants were currently living with their partner. This was used instead of marital status, which provided limited information on relationship status in this sample as the majority of women reported having never been married.

Highest educational level

Highest educational level was self-reported as none, GCSE or equivalent vocational qualification, A level or equivalent, first degree or higher degree. This was coded for analyses as none/GCSE, A level/vocational or degree level. GCSEs are public exams usually taken in school at 15–16 years; A levels are public exams usually taken in school at 17–18 years, and are generally required for University entrance.

Household income

Household income (participant and partner, pre-tax per year) was recorded as <£12,688, £12,688–£17,628, £17,629–£23,452, £23,453–£32,500 or > £32,500.

Index of multiple deprivation

Index of multiple deprivation [27] is assessed at the lower layer super output area (LSOA) level. Women were given an index of multiple deprivation score based on their postcode. Based on data for all LSOAs in England and Scotland, each LSOA was defined based on its quintile group [28].

Parity

Parity was self-reported and defined as the number of previous pregnancies going beyond 20 weeks’ gestation.