I've read a couple articles that touch on this. The current theory has been summarized as there being an evolutionary arms race between genes that benefit the

I've read a couple articles that touch on this. The current theory has been summarized as there being an evolutionary arms race between genes that benefit the child at the mother's expense and genes that preserve the mother while being somewhat detrimental to the child's fitness (but as a result beneficial to her other offspring). In this regard, longer gestation and a larger cranium are both better for the child. But our large crania are difficult to fit through the various choke points in the birth canal, and endanger the mother's life. The result is basically gestation as long as possible and crania as large as possible to permit natural childbirth to usually be non-fatal -- but that inflection point is one where even a non-fatal childbirth is difficult and painful. (There are also elements of the theory that relate to frequency of fertility, and why human menstrual bleeding far exceeds that of other mammals.) Because of the external gestation, my understanding is that survival rates for marsupial offspring are lower than for mammals. As for other mammals, giving birth is difficult for them too, though we don't have a way to measure how their pain compares to ours. However, no other mammal has gone down an evolutionary pathway where its fitness depends on brain size, so no other mammal has to contend with evolutionary pressures that so directly affect the ease of birth. (Plus, bipedal mammals with upright gaits are rare, so the evolutionary pressure affecting pelvic shape are different.)