Why Do Women Have Twins?

From Science:

Twinning Giving birth to twins is rough, especially in rural regions. They tend to be born smaller and weaker than single babies, and their mothers have more complications during childbirth. So why did twinning evolve? A new study in Gambia finds that women who have twins also tend to have single babies that are heavier than average at birth, which makes them more likely to survive. Since the 1950s, the U.K. Medical Research Council has been collecting data and providing medical care in Gambia. It's a highly unusual data set, says evolutionary anthropologist Rebecca Sear of Durham University in the United Kingdom, with a length and thoroughness that's “unheard of for populations without good access to medical care.” Evolutionary biologist Ian Rickard of the University of Sheffield in the United Kingdom wondered whether the data could shed light on the biology of twins. Rickard and colleagues looked at the birth weights of 1889 single babies born to Gambian women over a 30-year period. Then they examined which of these mothers also had twins. Single babies born after twins were 226 grams heavier on average than single babies whose mothers had no twins, the team reports today in Biology Letters. This wasn't surprising, Rickard says, because carrying twins is thought to improve blood flow to the uterus and “prime” it for later children, allowing them to more easily receive nutrients. What did surprise the researchers was the discovery that when single babies were born before twins, the singles tended to be 134 grams heavier than average.

Both findings suggest that the disadvantages of having twins balance out when a mother gives birth to additional children, who tend to be larger and healthier on average than children born to mothers who don't have twins, Rickard says. And that may explain why twinning has stuck around.

More here.