Grow or pass: the dilemma of our shoulders during childbirth

The human head is not our only possession to be born immature and then blossom in early childhood: our shoulders too! According to Japanese works published in PNAS, the growth of our shoulders slows down during fetal life and then accelerates after birth. A biological trade-off facing the dilemma between facilitating childbirth and providing people with the broad shoulders that are likely needed for locomotion and breathing.

The shoulders sometimes remain locked during childbirth.

Each species has its own survival strategy. In humans, our expensive and bulky brains are our greatest asset. Unfortunately, the mothers’ pelvis could not keep up with the same growth, presumably so as not to affect their motor abilities or to better hold the abdominal organs and the fetus during pregnancy. For this reason, our newborns are born with small heads, very immature brains, and skulls that have not yet fused. As a result, the mass of the brain of a newborn human reaches only 30% of the mass of an adult, compared with 40% in chimpanzees and 60% in macaques.

But the skull would not be the only organ adapted to the mother’s birth canal. In any case, this is demonstrated by Japanese researchers who looked at the width of the shoulders. Several hypotheses explain why our shoulders should remain broad. They could serveto stabilize the body when walking upright and facilitate high-speed throws“, the researchers suggest. It is also likely that shoulder width is related to the size of the chest and, therefore, with good postnatal respiratory function.

Result, “prevalence of shoulder dystocia, i.e. locking of the shoulders of the fetus in the birth canal, relatively high in humans“Especially when the child is of considerable weight, they exhibit. However, maybe our shoulders have adapted for the best to stay narrow during birth. “We assume that in humans, intrauterine development of the shoulder joint is limited by obstetric reasons.‘, the researchers add.

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