But this causes the pelvic opening to be small. Intelligence requires large brains and thus large cranial sizes. These two conflicting features lead to the tight fit of the infant through the birth canal. Trevathan's thesis is that problems relating to human deliveries produced selective pressures which led to the nearly universal human practice of midwifery. This is not to say that midwifery is an absolute necessity.
Women have for millennia delivered babies all by themselves with absolutely no assistance.
The increase in mortality is what drove humans to use midwives in all cultures. There are numerous problems delivering breech babies. The buttocks, feet, or knees are poor dilators of the cervix, and, thus, labor and delivery are often prolonged. A common problem is that women feel the urge to push before the cervix has dilated completely. This could result in the fetal head being held too long at the cervix.