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