Health inequalities remain resistant to interventions that primarily target individual behaviour. Although systems approaches are increasingly promoted, their application in practice is often not well grounded in real-world settings. In this protocol paper, we present the approach we will take in an overarching project that synthesises the combined insights of four ongoing systems-based research projects on system-based approaches for reducing health inequalities in the Netherlands. By bringing together and comparing findings across diverse contexts, populations and interventions, we aim to generate an empirically grounded understanding of what works, for whom, in what contexts and why, and to derive actionable strategies for systemic change to reduce health inequalities.

We use a realist approach to synthesise insights from the four ongoing projects. The design involves four iterative steps: (1) Identifying cross-cutting themes from project proposals and literature, (2) Developing and refining context–mechanism–outcome (CMO) configurations through literature review and Slow Science meetings, (3) Engaging Critical Friends to co-develop actionable strategies and (4) Assessing and validating these strategies across diverse contexts. Iterative feedback loops ensure continuous refinement, integration of stakeholder perspectives and exploration of emergent challenges. This design enables theory-informed, practice-based strategies to support sustainable system change in reducing health inequalities.

Ethical approval for the four underlying projects has been obtained from the relevant institutional review boards, and the way their data is used for this overarching project falls within their approved scope. Dissemination will be ongoing and co-created with stakeholders, including policy briefs, factsheets, educational tools and academic publications, to support uptake of strategies for systems change.

To provide population-level insights into COVID-19 testing behaviour and test results among all pregnant women in the Netherlands and to assess the effects of SARS-CoV-2 infection during pregnancy on maternal and neonatal health outcomes.

Retrospective population-based cohort study.

Dutch registry data on maternal and neonatal health outcomes linked with COVID-19 testing and sociodemographic data for the study period 2020 and 2021.

To study testing behaviour, all pregnant women who gave birth in the Netherlands during 2020 and 2021 were included (N=322 720). To study the effects of maternal infection, women who gave birth between June 2020 and September 2021 and who were tested for COVID-19 were included (N=68 059).

For testing behaviour: number of COVID-19 tests performed and COVID-19 test results. For neonatal health outcomes: preterm birth, low birth weight for gestational age (small for gestational age (SGA)), BIG2 (preterm birth and/or SGA), Apgar score at 5 min below seven (low Apgar), Apgar score at 5 min below four (very low Apgar), neonatal intensive care unit admission, congenital anomalies and mortality. For maternal health outcomes: major postpartum haemorrhage (>1000 mL), severe ruptures (third or fourth degree), type of delivery and episiotomy.

Compared with the reference group (women aged 30–34), women under 20 had the lowest probability of being tested (16.5% vs 31.3%; OR 0.43, 95% CI 0.38 to 0.49), but when tested, they had significantly higher odds of testing positive (19.3% vs 12.9%; OR 1.62, 95% CI 1.21 to 2.14). Women originating from ‘other African’ countries were least likely to be tested (15.1%; OR 0.37, 95% CI 0.35 to 0.39), while women whose country of origin was ‘Morocco’ were most likely to test positive when tested (33.4%; OR 3.63, 95% CI 3.35 to 3.93). While over all trimesters a SARS-CoV-2 infection during pregnancy did not show significant effects, an infection during the first trimester was associated with an increased risk of preterm birth (5.2% vs 6.4%; OR 1.25, 95% CI 1.03 to 1.52) and a low 5-min Apgar score (1.9% vs 2.9%; OR 1.50, 95% CI 1.12 to 2.02). No significant adverse maternal health effects were observed.

There were significant differences in testing behaviour and the probability of testing positive for COVID-19 among pregnant women from different age groups, countries of origin and socioeconomic backgrounds. SARS-CoV-2 infection during pregnancy was not associated with significant effects on maternal health outcomes, and only limited effects on neonatal health were observed. Only infections occurring in the first trimester were linked to an increased risk of preterm births and low 5-min Apgar scores.