Search strategy
Four databases of published literature (MEDLINE ALL (Ovid), Embase (Ovid), Food Science and Technology Abstracts (Ovid) and CINAHL (EBSCOHost)) were searched from database creation to the cut-off date of March 5th2020. Both medical subject headings and text keywords were used to develop a comprehensive search strategy with inputs from the whole research team including the health science librarian (LS)26. Two groups of keywords were used: ‘preconception or pregnancy’, and ‘diet quality’ (See Table S1 for detailed search terms used in OVID Medline). The reference lists of included studies and relevant review articles were screened to identify any additional studies. When eligible abstracts were identified, the title and authors were searched to determine if full-text articles had been published. Eligibility was limited to full-text articles published in scholarly peer-reviewed journals in English27.