Why Have Twins?

Mothers more likely to have twins have heavier, healthier non-twin babies, possibly explaining why twinning evolved.

Aug 11, 2011
Jef Akst

FLICKR, JOELLE INGE-MESSERSCHMIDT

New research shows that a genetic propensity for multiples could result in heavier single babies, which tend to be healthier and more likely to survive, according to a study published Tuesday (August 9) in Biology Letters, possibly explaining why some women have twins, despite the inherent dangers involved.

Bearing two babies at once is risky business—they are often smaller and weaker than single babies, and there’s a greater chance that either the mother or the babies, or both, will not survive. So the fact that some people have twins has long stumped scientists. To see if he could find an answer, evolutionary biologist Ian Rickard of the University of Sheffield in the United Kingdom mined a unique data set of 50 years’ worth of medical data on people in Gambia, collected by the UK Medical Research Council. Examining the birth weights of nearly 2,000 single babies, Rickard and his colleagues found that those born to mothers who had also had twins were 226 grams heavier than mothers who never gave birth to multiples. Thus, bearing twins on occasion might also afford mothers to have healthier single babies, suggesting that the benefits to the mothers might outweigh the costs of twinning.

Indeed, carrying twins is believed to improve blood flow to the uterus, which could benefit future babies by providing better access to nutrients, Rickard told ScienceNOW. But the study found that even single babies born before their younger twin siblings tended to be heavier, suggesting other mechanisms must be at play. The authors suggest that the answer may lie in a protein called IGF-1, which in addition to increasing the chance of twinning (by causing the ovaries to release multiple eggs), regulates fetal growth.