Twin Birth Order, Birthweight and Birthweight Discordance: Any Relationship?
Aim: To assess the truthfulness of this common concept that first-born twins are usually heavier than their second-born siblings at birth.
Methods: In a series of 104 sets of live-born twins, the birth weights of first-born twins were compared with those of their second-born siblings, after controlling for gender. Their intra-pair birthweight differences were determined and twin pairs whose birthweight difference was 15% or more were designated as discordant.
Results: Twin I was heavier than Twin II in 61.5% of cases while Twin II was heavier than Twin I in 28.9% of cases. Twins I and II had equal birth weight in 9.6% of cases. Comparing the mean birthweight of the first-born-male twin with that of second-born- male twin, it was 2515 + 427g [95 % Confidence Interval, CI = 2402 - 2628) versus 2432 + 435g (95% C I = 2321 - 2543) p>0.05. The mean birthweight of first-born-female twin was 2326 + 445g (95% CI =2214 - 2439) while that of the second-born-female twin was 2325 + 501g (95% CI = 2197 - 2453) p>0.05. When the birthweight difference exceeded 750g, the probability that Twin I will be heavier than Twin II was 83.3% (5 of 6).
Conclusion: Although the first-born twin was more often heavier than their second-born siblings, either could weigh more or less at birth. The larger the birthweight difference between growth-discordant twin pair, the greater the probability that the heavier twin would be delivered first.
Keywords: Birth order relative birth weights, birthweight discordance, twins
Copyright on any research article in the Nepal Journal of Obstetrics and Gynaecology is retained by the author(s).
The authors grant the Nepal Journal of Obstetrics and Gynaecology a license to publish the article and identify itself as the original publisher.
Articles in the Nepal Journal of Obstetrics and Gynaecology are Open Access articles published under the Creative Commons CC BY-NC License (https://creativecommons.org/licenses/by-nc/4.0/)
This license permits use, distribution and reproduction in any medium, provided the original work is properly cited, and it is not used for commercial purposes.