Harvey Karp, the bestselling author of the “The Happiest Baby on the Block,” has some advice on his website for frazzled new parents: “Remember — your baby’s brain was so big that you had to ‘evict’ her after 9 months, even though she was still smushy, mushy and very immature.”

It’s not an idea unique to Karp. Scientists have long struggled to explain the myriad challenges attending human childbirth compared to other primates, from the relative helplessness of human infants, to the very “tight fit,” as some researchers have put it, between the female human pelvis and the typical size of a child that must pass through it.

The mystery was the catalyst for what became known as “the obstetrical dilemma,” a long-debated though widely accepted hypothesis suggesting that the upright gait of Homo sapiens was accompanied by a narrowing of the pelvis — an evolutionary tradeoff that resulted in increased risks to pregnant mothers as they struggled to push large-brained babies through ever-slimmer birth canals. Among other things, the dilemma has been used to suggest that the wider, birth-giving hips of women have hindered them locomotively and athletically — and perhaps even evolutionarily — compared to men.

That has always struck some scientists as too pat an explanation, though it is only in the last decade or so that the theory, which still has many subscribers, has received substantive pushback. Today, challenges abound for the idiosyncrasies of human gestation and birth — including new notions that look beyond evolution to more proximate and modern factors like poor diet and obesity.

Of course, rigorous debate over the relative strengths and weaknesses of theories in this cul-de-sac of physiological science will surely continue. But for all the back-and-forth, one thing seems quite clear: The days of simply describing the human birth process — and women themselves — as evolutionarily compromised seem to be coming to an end.

For some researchers, that change in thinking is long overdue.

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Sherwood Washburn, the physical anthropologist who coined the phrase “obstetrical dilemma,” first published his theory in the September 1960 issue of Scientific American. He argued that, “in man, adaptation to bipedal locomotion decreased the size of the bony birth canal at the same time that the exigencies of tool use selected for larger brains. This obstetrical dilemma was solved by the delivery of the fetus at a much earlier stage of development.”

Early delivery, he concluded, foisted far greater responsibility on the “slow-moving mother,” who was now forced to hold her “helpless, immature infant,” while the men went out hunting.

The assumption that “women are compromised bipedally in order to give birth,” is widely accepted says anthropologist Holly Dunsworth of the University of Rhode Island. But Dunsworth sees flaws in this premise. Women already have a range of dimensions in their birth canal, she thought, and they are all walking just fine. Indeed, research on human skeletons by anthropologist Helen Kurki of the University of Victoria in Canada has shown that the size and shape of the human birth canal varies very widely, even more so than the size and shape of their arms.

So in 2007, Dunsworth went looking for evidence to support the obstetrical dilemma as it has traditionally been understood.

“When I couldn’t, I thought I was crazy,” she says. Intrigued, she enlisted Anna Warrener, a professor of biology and biomechanics, then at Harvard University, to test the notion that wider pelvises in women decrease the efficiency of locomotion. After measuring the chain reaction of forces moving through the body — from the foot to the leg to the hip — Warrener and her colleagues found that wider hips do not increase the cost of locomotion. Indeed, both women and men are equally efficient at walking and running, and in hunter-gatherer societies, women walk, on average, 5.5 miles per day, often while caring and feeding infants as well.

“The obstetric dilemma, in its definition, has housed this idea that women aren’t as good as men in some things because they have to give birth,” adds Cara Wall-Scheffler, an evolutionary anthropologist who studies human locomotion at Seattle Pacific University. “I have a number of papers that show that women are great walkers, and in some particular tasks women are better — they don’t use as much energy, they don’t build as much heat, they can carry heavier loads with less of an energetic burden.”

Dunsworth has an alternative theory as to why human pregnancy ends when it does: It’s called EGG, for “energetics of gestation and fetal growth,” and it applies not just to humans but to other mammals too. While a mother’s metabolic rate doubles during pregnancy, the fetus’s energy needs to increase exponentially towards the end of pregnancy. “As the fetus gets bigger and bigger and costlier and costlier to grow inside of the uterus,” Dunsworth explains, the mother’s metabolic rate reaches a limit. But the baby has to continue growing, “so the only way to do that is to get born.”

She is currently testing EGG on pregnant marmosets, measuring their energy use and metabolic rate during pregnancy over time, “to see if they give birth when they reach their maximum sustainable metabolic rate, as we do.”

Still others seek to explain why human brain volume has tripled over the past 2.5 million years, from the time of the Australopithecines. In a 2016 paper, brain and cognitive sciences researchers Steven Piantadosi and Celeste Kidd of the University of Rochester argue that helpless, larger-brained but early-born babies select for parents with advanced intelligence who must interpret their wordless signals; these larger-brained parents produce babies with ever-increasing brain size, a self-reinforcing process leading to “runaway selection for premature infants and big brains.”

Dunsworth readily acknowledges that childbirth can be difficult, and that the human birth canal is indeed a tight fit for the fetus, even though humans are born with the smallest relative brain of all primates (only 30 percent of our adult brain size, compared to chimps, whose brains at birth are 40 percent of adult size.) Globally, an average of 216 women die for every 100,000 live births, according to data from UNICEF. But the disparity between high- and low-income countries is gigantic: The lifetime risk of maternal death in rich countries is 1 in 3,300, compared to 1 in 41 in poor countries.

As such, blaming reproductive complications on evolution, writes Pamela K. Stone of the Culture, Brain, and Development Program at Hampshire College in Amherst, Massachusetts “conceals the larger health disparities and risks that women face globally.”

Childbirth is difficult for many reasons, she writes — among them the 19th century switch from birthing in the upright position, which allows the pelvic girdle to expand in response to contractions, to the supine position (still common among women in the West) which often requires the use of forceps.

Enter Jonathan Wells, a professor of anthropology and pediatric nutrition at the Great Ormond Street Institute of Child Health at University College London who argues for a competing hypothesis on the obstetrical dilemma. For starters, Wells argues, long-term ecological trends have likely played a role in changes in both pelvic dimensions and offspring brain size. One such trend was the rise of agriculture about 11,500 years ago in the Levant, which led to a shift from high-protein diet common among foragers to one replete with cereals. A high-carb diet is associated with both increased birth weight and shorter stature in the mother, and short stature is linked to smaller and flatter pelvises.

By that reasoning, the emergence of agricultural diets could have impacted “maternal mass and brain size, and may therefore have exacerbated the obstetric dilemma,” he says.

More recently, Wells has pointed to trends in both malnutrition and obesity as culprits in what he describes as a “new” obstetrical dilemma. According to Wells, this “dual burden” is contributing to a rising toll of obstructed labor, gestational diabetes, and larger-than-average newborns. Wells describes his theory in the April 2017 issue of The Anatomical Record.

Between 1980 and 2013, the percentage of overweight and obese women globally rose from 29.8 percent to 38 percent. At the same time, one in three people are malnourished in one form or another. “There is rapidly accumulating evidence,” Wells says, “that the dual burden of malnutrition can occur within the same individuals: Those who experienced poor nutrition and became stunted in early life, but who have also been exposed to obesogenic pressures from childhood onwards and who have therefore gained excess weight subsequently.”

As Wells notes, obstructed labor, where delivery of the baby causes harm to the mother, child or both, accounts for 12 percent of maternal mortality worldwide. It also increases substantially the risk of serious long-term maternal injuries, such as obstetric fistula. Dunsworth’s EGG theory can’t explain this frequency, he says.

But the combination of obesity and malnutrition can: Malnutrition and infectious disease in childhood is linked to short stature, which is associated with smaller pelvises in adulthood. Obesity, which is rising fastest in populations most prone to childbirth complications, increases the risk of delivering a “macrosomic” baby, whose birth weight exceeds the 90th percentile in any given population. “Overweight women in most populations are more likely to develop gestational diabetes if they are also short,” Wells adds. The combination of gestational diabetes and maternal obesity doubles the risk of macrosomic babies. So in theory, Wells says, a short overweight woman has two different risk factors for obstructed labor: smaller pelvic dimensions, and a higher probability of producing a large newborn.

This scenario is further aggravated by the persistence of child marriage, in which teens give birth before pelvic growth is completed, and gender inequality. A recent study of 31 countries in sub-Saharan Africa conducted by Alissa Koski, a postdoctoral scholar at the UCLA Fielding School of Public Health, found that more than one-third of girls in more than half of the countries studied married before the age of 18. In another study of 96 countries, Wells and his colleagues found “strong associations” between societal gender inequality and the prevalence of low birth weight, stunting, wasting, and child mortality. “On this basis,” he says, “societies with high levels of gender inequality are more likely to produce adult women of smaller body size,” which will impact the dimensions of the pelvis.

At the other extreme, he notes, obesity is increasing in prevalence faster in women than in men. Given these rapid increases in obesity, overweight women are more likely to experience difficulties in delivering babies if they were also stunted in childhood, Wells predicts — although so far, he doesn’t have the data to prove it. It is clear, however, that caesarean delivery has become one of the most common surgical procedures worldwide, increasing to “unprecedented levels” between 1990 and 2014 and ranging from 6 to 27 percent of all births in the least to most developed regions, respectively.

Dunsworth sees this as something of a self-fulfilling prophecy. “I worry that this idea [of the obstetrical dilemma] is justifying status-quo high rates of C-sections and other childbirth interventions,” Dunsworth says. “People say, ‘it’s just evolution — there’s nothing we can do, and here’s how technology helps, and that’s fabulous. But I know we’re overdoing it. Everybody knows that.”

While Dunsworth says she admires Wells’ research, she adds that she wishes he would come out a little more strongly against the evolutionary obstetrical dilemma.

For his part, Wells describes the work of Dunsworth and her colleagues as being of “major importance.” But “that doesn’t mean that Washburn had no important message,” he adds. “We have to acknowledge that the process of birth is surprisingly complex in humans, compared to other apes.”

“It is very clear from maternal mortality statistics that the contemporary burden of the obstetric dilemma is highly unequally distributed amongst women,” Wells says. “This suggests that if we had a better understanding of its biological basis, we could improve our efforts to reduce the burden of maternal and child mortality.”

CORRECTION: An earlier version of this piece imprecisely described the number of women who die annually due to complications from pregnancy or childbirth. The precise figure is 216 women for every 100,000 live births.

Josie Glausiusz is a journalist who writes about science and the environment for magazines including Nature, National Geographic, Aeon, and the Israeli newspaper Haaretz. Her Hakai magazine article, Land Divided, Coast United won Amnesty International Canada’s 2015 Online Media Award. She is the author of Buzz: The Intimate Bond Between Humans and Insects. Follow her on Twitter @josiegz.