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IntroductionPreterm birth is one of the main problems in obstetrics, and the most important cause of neonatal mortality, morbidity and neurodevelopmental impairment. Multiple gestation is an important risk factor for preterm birth, with up to 50% delivering before 37 weeks. Progesterone has a role in maintaining pregnancy and is frequently prescribed to prevent (recurrent) preterm birth and improve pregnancy outcomes in high-risk patients. However, little is known about its long-term effects in multiple gestations. The objective of this follow-up study is to assess long-term benefits and harms of prenatal exposure to progesterone treatment in multiple gestations on child development.Methods and analysisThis is a follow-up study of a multicentre, double-blind, placebo-controlled randomised trial (AMPHIA trial, ISRCTN40512715). Between 2006 and 2009 women with a multiple gestation were randomised at 16–20 weeks of gestation to weekly injections 250 mg 17α-hydroxyprogesterone caproate or placebo, until 36 weeks of gestation or delivery. The current long-term follow-up will assess all children (n=1355) born to mothers who participated in the AMPHIA trial, at 11–14 years of age, with internationally validated questionnaires, completed by themselves, their parents and their teachers.Main outcomes are child cognition and behaviourAdditional outcomes are death (perinatal and up to age 14), gender identity, educational performance and health-related problems. We will use intention-to-treat analyses comparing experimental and placebo group. To adjust for the correlation between twins, general linear mixed-effects models will be used.Ethics and disseminationAmsterdam UMC MEC provided a waiver for the Medical Research Involving Human Subjects Act (W20_234#20.268). Results will be disseminated through peer-reviewed journals and summaries shared with stakeholders, patients and participants. This protocol is published before analysis of the results.Trial registration numberNL8933.

What are the novel findings of this work?This follow-up study is the first to evaluate the effect on subsequent pregnancy of pessary use to prevent preterm birth in a multiple pregnancy within the setting of a randomized controlled trial. We found no long-term effects of pessary use on the outcome of subsequent pregnancy and maternal quality of life 4 years after the index pregnancy. What are the clinical implications of this work?The findings of this study improve our understanding of the long-term maternal consequences of the use of a pessary and can help better inform clinicians and pregnant women.