If you’ve seen cats, dogs, cows, or horses give birth, you’d wonder how amazing it is for their babies to get on their feet and even walk around hours after they’re born. It is in contrast to human babies who are born completely dependent on their parents for food and care. And this is after spending nine months or so in the mother’s womb.

There are many theories about the reason behind the nine months of human pregnancy and this Live Science report explores the most popular ones.

Among the newest theories would be that of Holly Dunsworth, a University of Rhode Island anthropologist and researcher. She proposes that babies are born when they are because of the metabolic limitations of a human mother’s body. Dunsworth also tries to debunk the other theories concerning the duration of human pregnancy.

Fully Developed Baby Brains Won’t Fit in Mother’s Pelvis

When babies are born, they only bear around 30 percent of the adult human brain, which is less than the chimpanzee babies’ 40 percent brain at birth. To let the human brain develop into its full size, it would take 18 to 21 months of gestation period, as written by Adolf Portmann, zoologist and author of the book “A Zoologist Looks at Humankind.”

However, theories suggest that a fully developed baby brain would be too big for mom’s pelvis, calling it an “obstetrical dilemma.” It could be that human nature compromised between the size of the baby brain and the mom’s pelvis.

Dunsworth’s research, on the other hand, showed that the human gestation period is even longer than is expected, human baby brains are 47 percent bigger than baby gorilla brains, and human babies are born two times bigger than gorilla newborns. These only prove that nine months is quite enough, or even longer, to let humans grow in the mother’s womb.

The Obstetrical Dilemma

Dunsworth also addressed the problem claimed to be presented by the mom’s pelvis. However, their study did not present any connection between the size of the hips and the energy needed for them to walk or run. She said that a wider pelvis does not necessarily make walking or running more difficult.

And compared to a chimpanzee brain that reaches 40 percent of its adult size upon birth, this meant that mom’s pelvis only have to expand by 1.18 inches more to accommodate 40 percent of the human baby brain. Even this much extra width is within the normal range and won’t cost as much energy on the part of the mother.

Dunsworth explained that the answer behind the nine months of human pregnancy lies in metabolism rate needed to help the growing fetus. When mom hits the sixth month, they need two times the normal energy to maintain their body’s metabolic process and this increases to 2.1 times average at her ninth month. They often need more energy to sustain the developing fetus inside their womb. The highest metabolic rate humans can keep up with ranges from 2 to 2.5 times the normal rate. This means that after nine months, the mom is at her limit when it comes to expending the energy she needed to support her fetus.

Impact of Head Size and Pelvis

While Dunsworth’s research disputed that the nine-month gestation period is not determined by the human pelvis’ ability to accommodate a baby’s bigger brain and head, New Mexico State University biologist Wenda Trevathan would beg to disagree. Trevathan’s research showed that shoulder shape and size still have somewhat of an impact on how the baby gets out of the birth canal, which would still call for assistance.

John Fleagle, an evolutionary biologist of Stony Brook University School, sees human babies’ helplessness as a positive thing. He said that the baby brain’s smaller capacity at birth can help them adapt to their surroundings better with experience. This gives the babies time to get used to the environment that they will grow up in.

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