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Abstract
It is well established that boys are born heavier and longer than girls, but it remains unclear whether birth size in twins is affected by the sex of their co-twin. We conducted an individual-based pooled analysis of 21 twin cohorts in 15 countries derived from the COllaborative project of Development of Anthropometrical measures in Twins (CODATwins), including 67,850 dizygotic twin individuals. Linear regression analyses showed that boys having a co-twin sister were, on average, 31 g (95% CI 18 to 45) heavier and 0.16 cm (95% CI 0.045 to 0.274) longer than those with a co-twin brother. In girls, birth size was not associated (5 g birth weight; 95% CI −8 to −18 and −0.089 cm birth length; 95% CI −0.202 to 0.025) with the sex of the co-twin. Gestational age was slightly shorter in boy-boy pairs than in boy-girl and girl-girl pairs. When birth size was standardized by gestational age, the magnitude of the associations was attenuated in boys, particularly for birth weight. In conclusion, boys with a co-twin sister are heavier and longer at birth than those with a co-twin brother. However, these differences are modest and partly explained by a longer gestation in the presence of a co-twin sister.
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1 Department of Social Research, University of Helsinki, Helsinki, Finland; Department of Genetics, Physical Anthropology and Animal Physiology, University of the Basque Country UPV/EHU, Leioa, Spain
2 Department of Social Research, University of Helsinki, Helsinki, Finland; Institute of Clinical Medicine, University of Eastern Finland, Kuopio, Finland
3 Department of Public Health Nursing, Osaka City University, Osaka, Japan
4 Department of Education, Mokpo National University, Jeonnam, South Korea
5 Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden
6 Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden; Pediatric Allergy and Pulmonology Unit at Astrid Lindgren Children’s Hospital, Karolinska University Hospital, Stockholm, Sweden
7 Department of Biological Psychology, VU University Amsterdam, Amsterdam, Netherlands
8 Institute for Molecular Medicine FIMM, Helsinki, Finland; Department of Public Health, University of Helsinki, Helsinki, Finland
9 Obesity Research Unit, Research Programs Unit, University of Helsinki, Helsinki, Finland; Endocrinology, Abdominal Center, Helsinki University Central Hospital and University of Helsinki, Helsinki, Finland
10 Department of Noncommunicable Diseases Prevention, Qingdao Centers for Disease Control and Prevention, Qingdao, China
11 Department of Health and Exercise Sciencies and Colorado School of Public Health, Colorado State University, Fort Collins, USA
12 Psychology and Neuroscience, Duke University, Durham, NC, USA
13 Department of Genetics, Physical Anthropology and Animal Physiology, University of the Basque Country UPV/EHU, Leioa, Spain
14 Health Behaviour Research Centre, Department of Epidemiology and Public Health, Institute of Epidemiology and Health Care, University College London, London, UK
15 Healthy Twin Association of Mongolia, Ulaanbaatar, Mongolia
16 Graduate School of Biomedical and Health Sciences, Hiroshima University, Hiroshima, Japan; Healthy Twin Association of Mongolia, Ulaanbaatar, Mongolia
17 Bandim Health Project, INDEPTH Network, Bissau, Guinea-Bissau; Research Center for Vitamins and Vaccines, Statens Serum Institute, Copenhagen, Denmark; Department of Endocrinology, Odense University Hospital, Odense, Denmark
18 Department of Endocrinology, Odense University Hospital, Odense, Denmark
19 Department of Infectious Diseases, Odense University Hospital, Odense, Denmark
20 Department of Radiology, Semmelweis University, Budapest, Hungary; Hungarian Twin Registry, Budapest, Hungary
21 Department of Health Science, Ishikawa Prefectural Nursing University, Kahoku, Ishikawa, Japan
22 Istituto Superiore di Sanità - Centre for Behavioural Sciences and Mental Health, Rome, Italy
23 School of Epidemiology, Public Health and Preventive Medicine, University of Ottawa, Ottawa, Ontario, Canada
24 École de psychologie, Université Laval, Québec, Canada; Institute of Genetic, Neurobiological, and Social Foundations of Child Development, Tomsk State University, Tomsk, Russian Federation
25 Département de psychologie, Université du Québec à Montréal, Montréal, Québec, Canada
26 École de psychologie, Université Laval, Québec, Canada
27 École de psychoéducation, Université de Montréal, Montréal, Québec, Canada
28 The Australian Twin Registry, Centre for Epidemiology and Biostatistics, The University of Melbourne, Melbourne, Victoria, Australia
29 The Australian Twin Registry, Centre for Epidemiology and Biostatistics, The University of Melbourne, Melbourne, Victoria, Australia; Department of Epidemiology, School of Public Health, Seoul National University, Seoul, Korea
30 Department of Psychology, University of Minnesota, Minneapolis, MN, USA
31 Murdoch Childrens Research Institute, Royal Children’s Hospital, Parkville, Victoria, Australia; Department of Paediatrics, University of Melbourne, Parkville, Victoria, Australia
32 MRC Integrative Epidemiology Unit, University of Bristol, Bristol, UK
33 King’s College London, MRC Social, Genetic & Developmental Psychiatry Centre, Institute of Psychiatry, Psychology & Neuroscience, London, UK
34 The Hebrew University of Jerusalem, Jerusalem, Israel
35 Hadassah Hospital Obstetrics and Gynecology Department, Hebrew University Medical School, Jerusalem, Israel
36 Michigan State University, East Lansing, Michigan, USA
37 Centre of Human Genetics, University Hospitals Leuven, Leuven, Belgium
38 Centre of Human Genetics, University Hospitals Leuven, Leuven, Belgium; Department of Obstetrics and Gynaecology, Ghent University Hospitals, Ghent, Belgium
39 The Charles Bronfman Institute for Personalized Medicine, The Mindich Child Health and Development Institute, Icahn School of Medicine at Mount Sinai, New York, NY, USA
40 Novo Nordisk Foundation Centre for Basic Metabolic Research (Section of Metabolic Genetics), Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark; Department of Public Health (Section of Epidemiology), Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
41 Department of Social Research, University of Helsinki, Helsinki, Finland; Osaka University Graduate School of Medicine, Osaka University, Osaka, Japan