Larger male faetuses are associated with an elevated risk of dystocia

Dystocia means difficult or abnormal labour and childbirth.

  The word comes from Greek parts: dys (difficult) and tokos (birth). It is also known as "failure to progress" or dysfunctional labour. It affects about 20% of human pregnancies and can also occur in animals. 

Common Types

Labour dystocia: Slow opening of the cervix or slow movement of the baby down the birth canal.

Shoulder dystocia: The baby's head passes through, but one or both shoulders get trapped behind the pubic bone. 

Causes of Dystocia

Foetal factors: The baby is too large, in a breech or sideways position, or a shoulder gets stuck (shoulder dystocia).

Maternal factors: The birth canal or pelvis is too small, or the uterus does not contract strongly or regularly enough (uterine inertia).

The 'costly son' hypothesis: Why bigger sons may make births riskier across species

A new comparative review, published in The Anatomical Record, examines factors that may lead to birth complications among humans and other mammals. The study suggests that foetal sex may play an important role in risky births in humans and several other species.
It's often thought that human childbirth is unusually difficult among mammals. The idea is that an evolutionary "obstetrical dilemma" arose because of our bipedal pelvic anatomy and increasing brain size over time. Larger babies passing through a narrow birth canal can increase the chance of obstructed labour, affecting mortality for both the baby and mother. Yet difficult births, including cases in which a foetus is too large for the birth canal, occur across many placental mammals.

Some research has suggested that the obstetrical dilemma was solved by relatively earlier births, but other studies found no evidence that humans have shorter gestations than other apes, even after controlling for body size. Furthermore, researchers involved in the new study say there is a maximum foetus size compatible with safe delivery, and when it is exceeded, obstructed birth results. In other words, birth size and weight are known to be associated with difficult delivery.

The new study suggests, however, that because male babies also tend to be larger, male births tend to be riskier. The team says this pattern is not restricted to humans.

"Most taxa, including humans, displayed a bias toward larger male foetuses. Foetal sex has nonetheless not been incorporated in evolutionary perspectives of childbirth, perhaps because it is often assumed that the sex of each baby is determined at random, since most mothers are likely to produce both sons and daughters," the study authors write.

Every mother has the potential to gestate both offspring through the reproductive career. Hence, it might be assumed that selection has acted on offspring size in general to resolve the obstetrical dilemma. As this new research work shows, however, the risks of birth complications are not equal for sons and daughters.

Researchers compiled sex-specific birth-weight data for 140 placental mammal species across 11 mammal orders and compared sex differences among new-borns with adult body-size differences and developmental style. They then searched for reports of difficult birth by offspring sex.

This analysis led the authors to propose the "costly son" hypothesis, which states that larger male offspring may raise birth risks for both mothers and babies. They found that male new-borns were more often larger than female new-borns across many species. In humans, boys are, on average, heavier by around 3% to 4%, have bodies that are around 1% to 1.7% longer and have bigger heads at birth.

In particular, larger male new-borns were common in species in which adult males are larger than adult females. Species with male-biased adult size differences were about six times more likely to have larger male new-borns than species in which adult females are larger.

The research team says evidence from humans, livestock and some other mammals suggests male foetuses are more prone to difficult births (referred to as dystocia), especially when new-borns are relatively large compared with the mother. They note that data on birth complications by offspring sex are scarce outside humans and domesticated animals, leading to a focus on humans and domesticated animals rather than wild animals.

"Where sufficient evidence exists—principally in precocial domesticated species and some other ungulates—larger male foetuses are associated with an elevated risk of dystocia. In several primate species where dystocia is reported, male neonates have larger average anatomical dimensions than females," the researchers write.

Therefore, we can expect greater risk of obstructed labour, likely cephalopelvic disproportion, with male offspring in several taxa, but it has not yet directly been shown. The researchers cautiously interpret this as support for the 'costly son' hypothesis: male offspring disproportionately contribute to foetopelvic disproportion and obstructed labour owing to their larger average size at birth.

The researchers also point out that the results identify a small average shift in risk for male births, not a prediction for any individual pregnancy. Because this is a comparative review and analysis, it doesn't prove that foetal sex directly causes difficult birth, but it does provide a testable prediction.
Evolutionarily speaking, larger sons often have more offspring because of male-male competition, and thus more grandchildren for the mother, ultimately meaning reproductive "success." The Trivers–Willard hypothesis used this argument to explain why mothers might be more likely to produce sons or daughters, depending on their physical condition. The mother expends more energy on large male babies, and on male babies in general. This increases birth risk but also increases the chance of reproductive success later on.

The study authors explain, "In humans, male foetal sex imposes a double burden on mothers, combining increased obstetric risk with elevated metabolic investment. Mothers of sons expend more energy during pregnancy and lactation, and this higher energetic investment contributes to enhanced foetal growth and larger birth size. In turn, this increased size elevates the risk of obstructed labour and adverse birth outcomes."

The new study does not find that mothers are more or less likely to produce sons or daughters, but the researchers do say that younger mothers seem to produce smaller new-borns to partition energetic resources, regardless of sex. They also say it remains unknown whether females can adaptively adjust offspring sex to mitigate obstetric risk.

Future work could systematically test whether younger or first-time mothers adjust offspring sex ratios or mainly produce smaller babies.

Female foetuses are biologically smaller than male foetuses on average, both globally and in India.

 Across human populations, male foetuses tend to have a slightly greater crown-rump length and weight than female foetuses starting even in the first trimester. 

Male and female foetuses do not weigh the same, even in lower-income countries where pregnant women experience poor nutritional values.

Biological sex differences in foetal growth remain consistent globally: male foetuses are generally heavier and larger than female foetuses throughout pregnancy. However, maternal undernutrition alters how each sex grows and survives in the womb, often putting male foetuses at a much higher disadvantage.

The biological difference in weight remains because male and female foetuses utilize fundamentally different growth strategies in the womb when resources are scarce.

Males have a faster metabolic rate and rapid growth strategy. They require more energy and attempt to grow quickly regardless of the mother's nutritional intake. If nutrients are scarce, they invest energy into expanding the placenta to extract more food. This makes them highly vulnerable; if the mother's nutrition is severely deficient, male foetuses are more likely to suffer from growth restriction, fail to survive, or be miscarried. 

Female Foetuses (Conservative Growth Strategy). Females grow more slowly and adapt more easily to a mother's limited nutritional stores. In resource-poor environments, female foetuses conserve energy and buffer themselves against shortfalls, which heavily increases their survival rate compared to males.

So good nutrition or a bad one, male foetuses try to grow large and pose a greater risk according to this new research work.

Nicole D. S. Grunstra et al, The "costly son" hypothesis: Sons exacerbate obstetrical dilemmas in humans and other mammals, The Anatomical Record (2026). DOI: 10.1002/ar.70331