Why Sons May Put Mothers Through More Painful Labour: Scientists Decipher The Evolutionary Price Of Having A Son

For generations, families across India and much of the world have celebrated the birth of a son. Yet a new study suggests that from an evolutionary perspective, sons may come with hidden biological costs. Researchers now argue that male babies are often larger before birth and may make labour more difficult and riskier for mothers. The idea has been given a striking name: the "costly son hypothesis."

The research, published in American journal The Anatomical Record, challenges long standing assumptions about why childbirth is so difficult in humans and suggests that the sex of the baby may be an overlooked factor in maternal health.

Every day, hundreds of women around the world die from complications related to pregnancy and childbirth. While modern medicine has dramatically reduced those risks in many countries, labour remains one of the most physically demanding experiences in human life. Scientists have traditionally blamed human evolution itself.

According to Ministry of Health and Family Welfare data "nearly 25,000 mothers die every year during childbirth in 2020 down from 32,500 in 2014'. The UNICEF says "with the birth of 25 million children each year India accounts for nearly one fifth of the world's annual child births. India is the only large country in the world where more girl babies die than boy babies. The gender differential in child survival is currently 11 per cent.'

For decades anthropologists have pointed to what is known as the "obstetrical dilemma." Humans evolved to walk upright on two legs. Bipedal walking favoured a relatively narrow pelvis. At the same time, human evolution also favoured increasingly larger brains, producing babies with larger heads. Delivering a large headed infant through a comparatively narrow birth canal became a difficult biological balancing act. According to a report in the respected American journal Science, babies typically enter the pelvis sideways and must twist and rotate during labour to navigate the narrow passage from womb to world.

However, recent studies have begun questioning whether difficult childbirth is uniquely human. Researchers have found that obstructed labour and challenging births occur throughout the mammalian world. That realization prompted scientists to ask another question. Could the sex of the baby also influence birth difficulty?

In their new study, researchers Nicole Grunstra and Jonathan Wells compiled sex specific birth weight data from 140 placental mammal species spanning 11 mammalian orders. Their findings showed a remarkably consistent pattern. Male offspring were more commonly larger at birth than female offspring. The researchers concluded that because male babies are generally bigger, they may impose greater obstetric risks on both mothers and themselves by increasing the likelihood of obstructed labour and difficult deliveries.

The authors write that male offspring "impose greater obstetric risk on mothers and themselves by increasing the likelihood of fetopelvic disproportion and dystocia." Dystocia is the medical term for prolonged or difficult labour. Their findings suggest that fetal sex represents an underappreciated dimension of mammalian childbirth.

Evidence supporting this idea is already emerging from human populations. According to the Science feature, previous work cited by the researchers found that Nepalese mothers delivering boys had an 18 per cent higher chance of obstructed labour. Caesarean deliveries in India were also found to be 3 per cent more likely when mothers were carrying sons.

The researchers believe mothers may invest more energy into developing male fetuses, allowing them to grow larger before birth. Those larger male babies may later develop into larger and more competitive adult males capable of producing more offspring. From an evolutionary perspective this can provide reproductive advantages. However, those advantages come with a price. As the researchers note, the benefits of producing larger sons may be balanced by higher risks of birth complications and mortality.

The findings take on added significance in India, a country that sees roughly 25 million births every year and accounts for nearly one fifth of global childbirths. The scale of childbirth in India means that even modest changes in maternal health outcomes affect millions of families. India has made remarkable progress in reducing maternal mortality during the past decade. According to the Ministry of Health and Family Welfare, India's Maternal Mortality Ratio fell from 130 maternal deaths per 100,000 live births in 2014 to 2016 to 97 in 2018 to 2020. More recent government data reported a further decline to 93 maternal deaths per 100,000 live births in 2019 to 2021.

These improvements reflect better access to antenatal care, greater institutional deliveries and expanded maternal health services. Institutional births increased from 79 per cent to 89 per cent nationally according to government figures, while antenatal care coverage also improved significantly.

Yet India continues to wrestle with another challenge. Despite decades of awareness campaigns and legal safeguards, many families still exhibit a preference for sons. The country's sex ratio at birth has improved, rising from 899 girls per 1,000 boys in 2014 to 913 girls per 1,000 boys in 2021, but it remains skewed. Government data attribute the improvement to sustained interventions, although concerns over son preference continue to be discussed by demographers and public health experts.

This creates a striking paradox. The new evolutionary research suggests that male babies may be biologically more demanding for mothers to deliver. Yet across many societies, including parts of India, sons continue to be valued more highly than daughters because of social, cultural and economic factors.

The study does not suggest that having a boy is inherently dangerous, nor does it imply that fetal sex alone determines childbirth outcomes. Scientists caution that many factors influence labour, including maternal health, nutrition, pelvic size, medical care and fetal position. Still, the research opens a new avenue for understanding one of humanity's oldest biological challenges.

As Science quoted anthropologist Anna Warrener, "Natural selection doesn't care if it's easy. It cares if it works." The new findings suggest that evolution may have accepted painful and sometimes dangerous births as the price for producing larger sons who may enjoy reproductive advantages later in life.

For mothers, however, that evolutionary bargain can mean a longer, harder and potentially more painful journey through childbirth. After centuries of blaming only upright walking and bigger brains, scientists are now asking whether the baby's sex should also be part of the story.

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