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.)
A kangaroo leaves the pouch in around 8 months.
EEG suggests earlier born babies can have huge merits in intelligence evolution: https://www.scientificamerican.com/article/does-a-newborn-s-...
In what sense? Our production rate is substantially lower than most other animals; mothers-to-be are made weaker than other animals during the process; the threat of death in childbirth for both child/mother is hardly the lowest of other animals
We have a relatively high population now, but its hard to imagine that its due to an efficient breeding process; rather our survival rate post-birth, lifespan and our phenomenal ability to edit the environment to increase resource production to support higher populations are likely better explanations
Put a pair of rabbits in a room and a pair of humans, with all needs met. We're not even competitive