Human babies are born helpless and needy, a fact that anthropologists have long explained by pointing to the size of the female pelvis. If babies were born with bigger brains, the theory goes, they'd get stuck in the birth canal. Instead, they stop gestating before they grow too large, resulting in completely dependent newborns.

But the story may not be so simple, new research finds. A study published today (Aug. 27) argues that it's not the size of mom's pelvis that determines when baby is born, but her metabolism.

"There is not a unique pelvic constraint on gestation length and baby size," study researcher Holly Dunsworth, an anthropologist at the University of Rhode Island, told LiveScience. "There is a certain capacity a mother has metabolically, and once that capacity is reached, the baby is born."

Baby heads and pelvic width

Human babies are born underdeveloped compared with other primates: Our brains are less than 30 percent their adult size at birth, compared with around 40 percent for chimpanzees, our closest living ape relative. In fact, it would take a gestation length of 18 to 21 months instead of nine months for human babies' brains to reach that level of development, according to zoologist Adolf Portmann's book "A Zoologist Looks at Humankind" (Columbia University Press, 1990).

The problem of fitting baby's head through mom's pelvis is known as the "obstetrical dilemma." Anthropologists have theorized that evolution has made a trade-off between big baby brains and the narrow pelvises needed for bipedal walking, resulting in babies born earlier than the ideal.

But Dunsworth's math suggests a different interpretation. In fact, she said, when you take body size into account, humans aren’t cutting gestation short at all. After controlling for body size, human pregnancies are second in length only to orangutans' and 37 days longer, not shorter, than gorilla and chimpanzee pregnancies, Dunsworth and her colleagues report in the journal Proceedings of the National Academy of Sciences.

"We're actually gestating longer than you would predict," Dunsworth said.

Human mothers also invest a lot of energy in their babies in the womb. The researchers found that human baby brains are 47 percent larger than baby gorilla brains, the primate with the next-largest infants. Human newborns are also twice the size of gorilla newborns. Even when controlling for maternal body size, human babies are larger than expected. In other words, humans aren't growing our babies smaller than average; we're super-sizing them. [Procreation Station: 11 Odd Animal Pregnancies]

Hips and energy

Next, Dunsworth and her colleagues turned to the other side of the dilemma: Mom's hips. Again, they found little evidence to back up the assumptions of the obstetrical dilemma. Women's wider hips are not less energy-efficient than men's narrower pelvises, the researchers calculated.

"Within the normal range of variation in women and men, walking and running are not compromised by a wider pelvis," Dunsworth said.

What's more, to get human brains up to the chimpanzee level of 40 percent of adult size, the pelvis would only have to widen about 1.18 inches (3 centimeters), well within the normal range of variation of humans today, the researchers found. This extra space wouldn't add any extra energy burden, they wrote.

So why are babies born after nine months of gestation and not some other point? Dunsworth and her colleagues found that metabolism may hold the answer. By six months of pregnancy, women expend twice their usual energy keeping basic metabolic processes going, a burden that only gets greater as the fetus gets larger. The typical maximum metabolic rate humans can sustain is between 2 times and 2.5 times average (with some exceptions such as professional cyclists). That means the female body may simply not be able to cycle through enough energy to keep a pregnancy going more than nine months. [8 Weird Changes That Happen During Pregnancy]

A new story

The findings complicate the "fairly simple" story of baby brain size being set by mom's pelvic size, said John Fleagle, an evolutionary biologist at Stony Brook University School of Medicine in New York.

"This is the most thorough and thoughtful consideration of this issue that anyone's ever done," Fleagle, who was not involved in the study, told LiveScience.

The findings basically switch around the assumption that the demands of walking and running on mom's pelvis determine baby's head size and suggest that instead, mom's metabolism sets the pregnancy length and baby size and the pelvis adapts to fit, Fleagle said. It's also possible that before the invention of agriculture, humans didn't have the energy to grow babies quite so big, meaning labor and delivery may not have been as much trouble tens of thousands of years ago as they are today.

Pelvic and head size still play a role in the birth process, noted Wenda Trevathan, a biological anthropologist at New Mexico State University who studies childbirth and was not involved in the research. Shoulder size and shape may also influence how babies emerge from the birth canal, all meaning that unlike other animals, humans are better off when they have assistance at birth.

Ultimately, Fleagle said, it may be a mistake to think of helpless babies as an evolutionary negative. Being born before the brain is set allows human offspring to learn from experience.

"The helpless baby is a baby that grows up in an environment that it has to deal with," he said.

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