Article open access publication

Birth size and gestational age in opposite-sex twins as compared to same-sex twins: An individual-based pooled analysis of 21 cohorts

Scientific Reports, Springer Nature, ISSN 2045-2322

Volume 8, 1, 2018

DOI:10.1038/s41598-018-24634-2, Dimensions: pub.1103452291, PMC: PMC5908858, PMID: 29674730,



  1. (1) University of the Basque Country, grid.11480.3c
  2. (2) University of Helsinki, grid.7737.4
  3. (3) University of Eastern Finland, grid.9668.1
  4. (4) Osaka City University, grid.261445.0
  5. (5) Mokpo National University, grid.411815.8
  6. (6) Karolinska Institute, grid.4714.6
  7. (7) Karolinska University Hospital, grid.24381.3c
  8. (8) VU Amsterdam, grid.12380.38
  9. (9) Institute for Molecular Medicine Finland, grid.452494.a
  10. (10) Department of Noncommunicable Diseases Prevention, Qingdao Centers for Disease Control and Prevention, Qingdao, China
  11. (11) Colorado State University, grid.47894.36
  12. (12) Duke University, grid.26009.3d
  13. (13) University College London, grid.83440.3b
  14. (14) Healthy Twin Association of Mongolia, Ulaanbaatar, Mongolia
  15. (15) Hiroshima University, grid.257022.0
  16. (16) Bandim Health Project, grid.418811.5
  17. (17) State Serum Institute, grid.6203.7
  18. (18) Odense University Hospital, grid.7143.1, Southern Denmark Region
  19. (19) Hungarian Twin Registry, Budapest, Hungary
  20. (20) Semmelweis University, grid.11804.3c
  21. (21) Ishikawa Prefectural Nursing University, grid.443808.3
  22. (22) Istituto Superiore di Sanità, grid.416651.1
  23. (23) University of Ottawa, grid.28046.38
  24. (24) Tomsk State University, grid.77602.34
  25. (25) Laval University, grid.23856.3a
  26. (26) University of Quebec at Montreal, grid.38678.32
  27. (27) University of Montreal, grid.14848.31
  28. (28) University of Melbourne, grid.1008.9
  29. (29) Seoul National University, grid.31501.36
  30. (30) University of Minnesota System, grid.437349.e
  31. (31) Murdoch Childrens Research Institute, Royal Children's Hospital, Parkville, Victoria, Australia
  32. (32) University of Bristol, grid.5337.2
  33. (33) King's College London, grid.13097.3c
  34. (34) Hebrew University of Jerusalem, grid.9619.7
  35. (35) Michigan State University, grid.17088.36
  36. (36) Universitair Ziekenhuis Leuven, grid.410569.f
  37. (37) Ghent University, grid.5342.0
  38. (38) Icahn School of Medicine at Mount Sinai, grid.59734.3c
  39. (39) University of Copenhagen, grid.5254.6, KU
  40. (40) Osaka University, grid.136593.b


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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