Research involving over 8,000 participants across three nations indicates that women’s pelvises have become narrower over the last 150 years. While several factors contribute to this change, it offers fresh insights that could prompt researchers to reevaluate the so-called “obstetric dilemma,” which pertains to the evolutionary constraints surrounding pelvic size.

The exact drivers of this change and its implications for health remain unclear. Nonetheless, a continued reduction in pelvic size could heighten the likelihood of requiring cesarean sections.

Maciej Henneberg, from the University of Adelaide, Australia, and his team revisited an existing dataset featuring 1,247 Australian women born from 1900 to 1984. They discovered that pelvic width decreased by 0.42 mm annually. In Poland, the pelvic width of 3,486 women reduced by 0.47 mm each year from 1880 to 1970, while in a sample of 320 Mexican women, the decline was 0.42 mm annually from 1900 to 1970.

“I find this compelling, considering that these regions, despite differences in height, show similar evolutionary trends,” remarks Philip Mitter Ecker from the University of Vienna, Austria.

“The dataset is impressive,” adds Leah Betty from University College London.

Henneberg posits that these findings suggest modern medical practices diminish the effects of natural selection on human pelvises. Historically, a baby too large or a birth canal too narrow often resulted in fatal outcomes for both mother and child. However, the introduction of safe, effective cesarean sections has lessened this evolutionary pressure, potentially leading to further narrowing of the birth canal and pelvis.

Mitteroecker agrees that cesarean sections are influencing the evolutionary forces at play in human birth anatomy. He predicts this trend may result in even narrower pelvises, as highlighted in his 2016 analysis, stating it represents the latest chapter in a long-standing narrative.

“Midwifery has ancient roots, uniquely human,” he explains. For thousands of years, women have supported each other during childbirth. This cultural tradition has mitigated selection pressures on the pelvis and birth canal, demonstrating how our actions influence our biological evolution. “In a way, cesarean sections are an extreme manifestation of this,” Mitteroecker concludes.

However, Betty remains skeptical that cesarean sections are the primary cause of the recent changes in pelvic width. She highlights that humans have also experienced increased height during the same period, likely due to improved nutrition and healthcare rather than evolutionary gene alterations.

“Diet plays a significant role in determining pelvic structure,” Betty notes. When undernourished, developing bodies prioritize certain organs, particularly the brain, over others. With adequate nutrition now available, bodies may be reallocating resources, leading to variances in proportions. “Different body proportions are certainly a possibility,” she observes.

Understanding the factors behind these changes is crucial for comprehending the challenges of human childbirth. However, the nature of this dilemma is still debated. In a 2024 study, Mitter Ecker and colleagues suggested that pelvic floor mechanics could be a significant contributor to narrowing, potentially heightening pressure on the pelvic floor and elevating the risk of incontinence.

It’s also feasible that both factors are at play. An April study involving 31,000 participants associated wider pelvises with easier childbirth, yet identified slower walking speeds and increased risk of pelvic floor conditions.

The implications of this dilemma extend beyond childbirth. Betty notes that competitive sports during childhood may also influence pelvic size, emphasizing that various environmental factors, including temperature, can affect the pelvis. Other studies tie the evolving obstetric dilemma to rising obesity rates, which may result in larger babies. Ultimately, a combination of these factors likely explains the observed changes, prompting researchers to classify the issue as a “multifactorial pelvis” challenge.

A narrower pelvis poses health implications, complicating vaginal births and potentially increasing the need for cesarean deliveries. “Nobody knows how long it will take to reach a point where natural childbirth ceases to exist,” remarks Renata Henneberg, one of the researchers on this new study and Maciej Henneberg’s wife.

Conversely, a narrower pelvis may reduce the incidence of pelvic floor issues, which can be quite severe. Childbirth can have long-lasting, detrimental effects on women’s lives,” Betty cautions.

Predicting future trends is complex, given the multitude of interacting factors. While fewer children may lead to decreased injury risks, later childbirth may counteract this. “Many changes are occurring simultaneously,” she reflects.