A 140-species review puts fetal sex inside the obstetrical dilemma

Obstructed labour is a geometry problem. The authors hold that there is a maximum fetal size compatible with safe delivery, and that birth obstructs when it is exceeded ^[5]. That makes the dimensions, not the label, the thing worth reading. In humans, boys are on average about 3 to 4 percent heavier at birth, about 1 to 1.7 percent longer, and have bigger heads ^[8]. Set those against each other and the relative mass gap is roughly 1.8 to 4 times the relative length gap ^[18]: the extra mass is not simply extra length, which is the part that matters for a fetus clearing a canal.

The comparative backbone is the strongest piece of the work. Sex-specific birth weights for 140 placental mammal species across 11 orders, compared against adult body-size differences and developmental style ^[6], yield a consistent bias toward larger male newborns ^[7], and that bias tracks adult dimorphism: species where adult males outweigh adult females were about six times more likely to have larger male newborns than species where adult females are the larger sex ^[9]. That is a statement about size, measured across many taxa. It is not a risk estimate.

The risk half of the argument runs on a much thinner base. Evidence that male fetuses are more prone to dystocia comes from humans, livestock and some other mammals, and is strongest when newborns are large relative to the mother ^[13]. In primates where dystocia is reported at all, male neonates are the larger sex on average, and the clearest signal comes from precocial domesticated species and other ungulates ^[16]. The authors are explicit about the gap: they expect greater risk of obstructed labour, probably cephalopelvic disproportion, with male offspring in several taxa, "but it has not yet directly been shown," and they call their reading cautious support for the costly son hypothesis ^[15]. Sex-resolved birth-complication data barely exist outside humans and domesticated animals ^[14]. So the descriptive claim spans 11 mammal orders while the causal claim spans us and the animals we breed ^[19].

What this does to the obstetrical dilemma is additive rather than substitutive. Bipedal pelvic anatomy and growing brain size remain the standard account of why human birth is hard ^[2], and nothing in the review dislodges them. But difficult births, including cases where the fetus is too large for the canal, already occur across many placental mammals ^[3], and the proposal that humans solved the problem by giving birth earlier is contested, with other work finding no shorter gestation in humans than in other apes after controlling for body size ^[4]. Into that unsettled picture the review inserts a variable that had been averaged away because fetal sex was treated as random, on the reasoning that most mothers gestate both sons and daughters over a reproductive career ^[11]. The authors' point is that the average conceals unequal risk between them ^[12].

The thing this does not tell you is anything about a given pregnancy. The authors say plainly that they have identified a small average shift in risk for male births, not a prediction for any individual case ^[17].