Photo/Illutration Newborns are kept in a hospital in Chuo Ward, Fukuoka, in March 2008. (Asahi Shimbun file photo)

A team of researchers at Kyoto University and other institutions found a mechanism that slows shoulder growth in fetuses, lowering the risk of a difficult labor.

A broad shoulder is required for humans to walk on two legs, for example, or throw something far for hunting, according to the team.

However, if a fetus's shoulder is too broad, it could get stuck during labor as a mother's pelvis is relatively small, and the birth canal is narrow.

The finding was published on April 12 in the Proceedings of the National Academy of Sciences of the United States of America, the official journal of the National Academy of Sciences.

The team used a CT scan to obtain images of 81 skeletons from humans ranging from a fetus to an adult.

The researchers found the growth of a fetus's collarbone slows before their birth.

They believe this is how babies make their bodies narrower so they can smoothly move through the birth canal.

The collarbone determines the shoulder breadth.

The team also found that once babies are born, collarbone growth quickens.

The researchers found this is a mechanism that species such as chimpanzees and snow monkeys don’t have.

The fetus's head can also cause a difficult labor. However, it has already been found that the skull is soft until babies are born, lowering the risk of a difficult labor.

Until this finding, however, it was unknown whether fetus's shoulders have a similar mechanism to reduce the risk of a difficult labor. 

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