Boys With Co-Twin Sisters Are Bigger at Birth

It’s long been known that boys tend to be bigger than girls at birth. But a recent multinational study of dizygotic twin pairs looked into whether birth size in twins is affected by the sex of the co-twin. The study was published in Scientific Reports. Scientific Reports is an online journal published by the Nature Publishing Group, covering all areas of the natural sciences. The researches did an analysis of 21 twin cohorts in 15 countries. 67,850 dizygotic twins were included in the study. Twins with a birth weight below 500 grams (1lbs, 1oz) and above five kilos (11lbs) were excluded from the study.

Risk of preterm delivery dependent on gender

The researchers found that twin boys having a co-twin sister were, on average, 31 grams heavier (1oz) and 0.16 centimeters (0.06 inch) longer than those with a co-twin brother. The researchers also found that the differences could be partly explained by a longer gestation in the presence of a co-twin sister. In girls, birth size was not associated with the sex of the co-twin. The researchers also compared the risk of preterm delivery between boy-boy, boy-girl and girl-girl twins. They found that compared to boy-boy twins, the mean gestational age was greater in both boy-girl and girl-girl twins. Both boy-girl and girl-girl pairs showed a lower risk of delivery before 37 weeks and 34 weeks.

Countries not equally represented

The researchers concluded that the main strength of the study was that it had a large sample size. They pointed to the fact that a generalization for the global population was not possible, because countries and regions were not equally represented. The participants in the study weighted heavily towards European ancestry populations following westernized lifestyle.