Of all mammals, humans are the only primates that need to seek midwifery during childbirth. For human women, childbirth is a long and painful process, in some cases even fatal. The World Health Organization estimates that about 830 women die every day due to complications that occur during pregnancy and childbirth.
How strange is the human birth canal?
The reason for this is related to the structure of the birth canal of human women, first of all, the human birth canal is relatively narrow, and it is estimated that in the 1960s, the global probability of newborns not being able to pass through the birth canal smoothly was 3%, that is, 30 times in every 1000 births, the newborn had difficulty passing through the birth canal.
The second is that the human female birth canal has a distorted structure, and the upper or entrance of the human female birth canal is round and transverse (from left to right) oval, which is considered the ideal condition for childbirth, but it is not clear why the lower part of the birth canal is obviously longitudinal (from front to back) oval, this twisted structure usually requires the fetus to rotate its body through the narrow birth canal.
To put yourself in your mother's shoes and understand your mother's childbirth process, imagine that you are about to be in the pot. When the fetus enters the birth canal, because the entrance is circular and transverse (from left to right) oval, it is most likely that he is head down, facing your side.
However, because the exit is longitudinal, he must turn his face towards your back and the back of his head against your pubic bone in the middle of the way. At this point, his shoulders are facing the left and right sides of your body. While the fetus is still inside you, he is still facing backwards, but then he will turn his head slightly to one side, and this rotation will help the fetus's shoulders to shift so that it can adapt to changes in the birth canal exit.
However, the birth canal of the human mother is closely consistent with the size of the fetus. The average pelvic opening in human women, the maximum diameter is 13 cm, the minimum is 10 cm, while the average length of the fetal skull from front to back is 10 cm, and the shoulder span is 12 cm. For the vast majority of mothers and fetuses, the twisted birth canal increases the difficulty and danger of childbirth.
The evolution of such a birth canal is also unique among mammals, and we all know that human reproduction and evolution are guaranteed by the continuation of future generations. However, human fertility has always been plagued by childbirth. There is clearly a contradiction between the two. Scientists have always wanted to solve this evolutionary puzzle.
Why is the birth canal so narrow in human women
First, the scientific community already has an answer as to why the human female birth canal is so narrow, and that is that the relatively narrow birth canal may be the result of human weighing childbirth, internal organ support, and upright walking.
Two million years ago, as humans evolved, they lost more ape traits such as short bodies, long hands, and small brains. Humans are taller, have shorter arms and have larger brain volumes. The early Homo erectus brain capacity was 800 ml, and the late Homo erectus brain capacity has reached 1200 ml, which is not far from the modern human brain capacity of 1350-1500 ml. However, this change in brain volume has affected women to give birth to fetuses, and the birth canal of human women was previously adapted only to fetuses with smaller brain volumes.
The increase in human brain volume means that from birth human babies have a much larger brain volume than other mammals, which leads to evolutionary conflicts with their own structure. On the one hand, women have to maintain narrow pelvis and birth canal in order to walk upright, but on the other hand, the huge head of the fetus makes it more difficult to pass through the narrow birth canal.
To reduce the risk of childbirth, humans have evolved shorter gestational periods that allow babies' heads to safely pass through the birth canal, but this has also led to a consequence: babies come to life before their brains are fully developed. Newborn babies develop brains that are only a quarter the size of adults, and newborns have a brain volume of only 400 milliliters and can reach about 1,400 milliliters as adults. The brain has matured to 97% when the baby is three years old, and when the baby is 5 years old, the size and capacity of the brain are very close to that of adults, and the corresponding structure of brain function is basically established such as memory, learning, movement control, etc.
Because the brain is not fully developed, so the fetal skull is not completely healed before and after birth, and can change shape when squeezed, which can make the fetus easy to pass through the female birth canal during childbirth, the most emblematic feature is the fontanelle, which is left at birth to delay the healing of the brain bone slit.
Therefore, this is a two-pronged move that humans have achieved by adapting to natural selection in evolution, both to make childbirth go more smoothly and to allow the brain capacity to grow larger after birth.
Why the birth canal of human women is distorted
So why does the birth canal of human women show a distorted structure? Recently, katya Stansfield of the University of Vienna in Austria and co-authors published in the journal BMC Biology provide new insights into why humans have evolved such complexly shaped birth canals.
The team believes that the support function of the pelvic floor muscles may have influenced the evolution of the shape of the human birth canal. After extensive biomechanical modeling of the pelvic floor, they found that deformation, stress, and strain were greatest when the pelvic floor was circular or transversely elliptical, while the longitudinal elliptical extension enhanced the stability of the pelvic floor. This is why the entrance to the human birth canal is transversely (from left to right) oval, while the lower part of the birth canal is distinctly longitudinal (front to back) elliptical.
So why didn't the entrance evolve into a longitudinal oval? The scientific community believes that the evolutionary trend of lateral extension of the pelvic entrance is limited by the amplitude of the body before and after when balancing the upright posture, rather than due to the efficiency of bipedal movements, the longitudinal deeper mouth requires a greater pelvic tilt and lumbar spine protrusion forward, which will endanger the health of the spine and the stability of the upright posture, and the different requirements of women for the entrance and exit of the pelvis may lead to the evolution of the curvature of the birth canal. This is a compromise between the development of human structure and evolution.
After reading this article, you should know how hard and risk it takes for women to give birth, so we should be considerate of our wives and mothers, they are great.