Review Relevance of individual participant data meta-analysis for studies in obstetrics: delivery versus expectant monitoring for hypertensive disorders of pregnancy Kim Broekhuijsen a, *, Thomas Bernardes b , Gert-Jan van Baaren c , Parvin Tajik d , Natalia Novikova e , Shakila Thangaratinam f , Kim Boers g , Corine M. Koopmans a , Kedra Wallace h , Andrew H. Shennan i , Josje Langenveld j , Henk Groen b , Paul P. van den Berg a , Ben Willem J. Mol k , Maureen T.M. Franssen a a University of Groningen, University Medical Center Groningen, Department of Obstetrics and Gynaecology, The Netherlands b University of Groningen, University Medical Center Groningen, Department of Epidemiology, The Netherlands c Department of Obstetrics and Gynaecology, Academic Medical Center, University of Amsterdam, The Netherlands d Department of Clinical Epidemiology, Biostatistics & Bioinformatics, Academic Medical Centre, University of Amsterdam, The Netherlands e Department of Obstetrics and Gynaecology, Frere Hospital, Walter Sisulu University, South Africa f Women’s Health Research Unit, Centre for Primary Care and Public Health, Queen Mary University of London, United Kingdom g Department of Obstetrics and Gynaecology, Bronovo Ziekenhuis, The Netherlands h Department of Obstetrics and Gynaecology, University of Mississippi Medical Center, United States i Division of Women’s Health, King’s College London Women’s Health Academic Centre Kings Health Partners, United Kingdom j Department of Obstetrics and Gynaecology, Atrium Medical Centre Parkstad, The Netherlands k Department of Obstetrics and Gynaecology, The Robinson Research Institute, School of Paediatrics and Reproductive Health, University of Adelaide, Australia European Journal of Obstetrics & Gynecology and Reproductive Biology 191 (2015) 80–83 A R T I C L E I N F O Article history: Received 25 February 2015 Received in revised form 16 May 2015 Accepted 19 May 2015 Keywords: Individual patient data meta-analysis Hypertensive disorders of pregnancy Preterm birth Maternal morbidity/mortality Perinatal morbidity/mortality A B S T R A C T Like many other research subjects in obstetrics, research on immediate delivery versus expectant monitoring for women with hypertensive disorders of pregnancy faces certain challenges when it comes to interpretation and generalisation of the results; relatively rare outcomes are studied, in a clinically heterogeneous population, while the clinical practice in some countries has dictated that studies in term pregnancy were completed before earlier gestational ages could be studied. This has resulted in multiple smaller studies, some studying surrogate outcome measures, with different in- and exclusion criteria, and without enough power for reliable subgroup analyses. All this complicates the generation of definitive answers and implementation of the results into clinical practice. Performing multiple studies and subsequently pooling their results in a meta-analysis can be a way to overcome the difficulties of studying relatively rare outcomes and subgroups with enough power, as well as a solution to reach a final answer on questions involving an uncertain and possibly harmful intervention. However, in the case of the current studies on delivery versus expectant monitoring in women with hypertensive disorders of pregnancy, differences regarding eligibility criteria, outcome measures and subgroup definitions make it difficult to pool their results in an aggregate meta-analysis. Individual patient data meta-analysis (IPDMA) has the potential to overcome these challenges, because it allows for flexibility regarding the choice of endpoints and standardisation of inclusion and exclusion criteria across studies. In addition, it has more statistical power for informative subgroup analyses. We therefore propose an IPDMA on immediate delivery versus expectant monitoring for hypertensive disorders of pregnancy, and advocate the use of IPDMA for research questions in obstetrics that face similar challenges. ß 2015 Elsevier Ireland Ltd. All rights reserved. * Corresponding author at: UMCG, HPC CB 21, PO Box 30 001, 9700 RB Groningen, The Netherlands. Tel.: +31 616972879. E-mail address: k.broekhuijsen@gmail.com (K. Broekhuijsen). Contents lists available at ScienceDirect European Journal of Obstetrics & Gynecology and Reproductive Biology jou r nal h o mep ag e: w ww .elsevier .co m /loc ate/ejo g rb http://dx.doi.org/10.1016/j.ejogrb.2015.05.023 0301-2115/ß 2015 Elsevier Ireland Ltd. All rights reserved.