Introduction Pregnant women with systemic lupus erythematosus (SLE) have an increased risk of maternal complications and adverse fetal outcomes. These include preeclampsia, preterm birth and fetal growth restriction. Interestingly, this increased risk persists in subsequent pregnancies, whereas it decreases in healthy women due to the development of maternal-fetal tolerance. As maternal-fetal tolerance is crucial for a healthy pregnancy, we hypothesize that its failure contributes to the increased risk of pregnancy complications in women with SLE. Therefore, we initiated the FaMaLE study to investigate the failure of maternal-fetal tolerance in pregnant women with SLE. Methods and analysis In the FaMaLE study, healthy women and women with SLE are included in their first trimester of pregnancy (< 14 weeks gestational age (GA)) at Amsterdam UMC. Throughout the pregnancy, data on SLE disease progression, pregnancy course, and medication use are collected. Peripheral blood is collected once per trimester, within 48 hours before delivery and 5-12 weeks post-partum. In addition, the placenta is collected after delivery. Whole blood, peripheral blood mononuclear cells (PBMC) and placenta samples are freshly analyzed by flow cytometry to assess immune cell composition. The resulting data are analyzed in relation to SLE disease course, pregnancy course and pregnancy outcomes. Ethics and dissemination The study has been approved by the Amsterdam UMC Medical Ethics Committee and all participating women will be asked to provide informed consent. The findings will be disseminated through peer-reviewed publications, presentations at scientific meetings and via patient organizations.

The authors have declared no competing interest.

This study was funded by FOREUM - Foundation for Research in Rheumatology, Amsterdam UMC and NVLE Foundation.

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Medical Ethics Committee of Amsterdam UMC gave ethical approval for this work

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All data produced in the present study are available upon reasonable request to the authors