Table 1 Inclusion/Exclusion criteria for systematic review

Include

Exclude

Population

Pregnant women

Intervention / mediator

Behavioural risk factors (e.g. smoking, alcohol). Social risk factors (e.g. Environmental (housing, working)). Maternal health status (both mental and physical health)

Genetic risks for preterm birth

Comparison across exposure

Comparison across socioeconomic strata (either individual or area-based)

Outcomes

Preterm birth and gestational age

Other birth outcomes (e.g. low birthweight)

Publication characteristics: Inclusion / exclusion criteria

Include

Exclude

Publication types

Primary studies from peer-reviewed literature, including those from reviews. Relevant secondary analyses (meta-analysis). Papers published or in-press. Working papers

Not primary research, e.g. letters, editorials, commentaries, conference proceedings, books and book chapters, meeting abstracts, lectures, and addresses. Previous reviews and meta-analyses, but relevant reviews were used to identify relevant primary studies

Types of study

Analytical techniques that are relevant to research question:

--Mediation

--Attenuation

Differential exposure

Other methods. Mediation or attenuation not specifically calculated within analysis

Year of publication

2000–2020

Language

English language