What counts as a backward birth differs from one species to another, though. Dr. Lawrence G. Barnes, curator of vertebrate paleontology at the Natural History Museum of Los Angeles County and a researcher who studies the evolution of whales, said that whale calves sometimes come out of the birth canal head first, rather than the proper tail-first presentation. That is fatal for the neonate because its face is exposed to the water as the rest of its body is still emerging, so it cannot get to the surface before drowning.

Danger also arises for human mothers when they give birth under severely unhygienic conditions, as they frequently did in the 18th and 19th centuries. At that point, male doctors had taken over the business of birth assistance from midwives, and they often brought to the mother's bedside infectious microbes from their nonpregnant sick and dying patients elsewhere.

Today, septic conditions and poor medical care plague pregnant women in many underdeveloped nations, resulting in huge numbers of unnecessary deaths. Last month, Unicef reported that 1 in 13 women in sub-Saharan Africa and 1 in 35 in South Asia dies of causes related to pregnancy and childbirth each year, compared with 1 in 3,200 in Europe, 1 in 3,300 in the United States and 1 in 7,300 in Canada.

The rapid growth of the human brain, or encephalization, in the last two million years or so has added to women's birth trauma. Yet the near-mismatch between the human pelvis and infant head is not the unique and severe problem that many imagine. It is true that among the great apes -- chimpanzees, orangutans and gorillas -- the female pelvis is quite roomy, and birth is far easier than it is for humans. But that is not the case for small species of monkeys, which have comparatively large brains without the bulk of adulthood seen in the apes. Among squirrel monkeys, for example, the fit of the birth canal is so tight relative to the baby that, as Dr. Stoller notes in her doctoral dissertation, labor obstruction is quite common. In one colony in Alabama, 16 percent of the infants are stillborn, and 34 percent of the young born alive die soon after birth, often of birth-inflicted injuries.

In shaping birth patterns, evolution seems to push to the limits of viability when there is a difficult problem to solve. Miles Roberts, head of the zoological research department at the National Zoological Park in Washington, said the most extreme ratio in primates of large infant size to narrow maternal birth passage was among the tarsiers, a small, insect-eating monkey found in Indonesia and the Philippines.

In this case, the fetus must grow a very large brain during gestation because it will have to start foraging for itself within a month. Insects are simply too energy-poor a food source to allow the mother to nurse the infant at length, as many primates do. "The infant has to get to work quickly in the three-dimensional space of an arboreal habitat," Mr. Roberts said. "That means it needs extensive motor coordination, which means it has to start out with a big brain." Tarsier birth is so difficult and improbable, Mr. Roberts said, that once the baby is out, "you can't imagine how the mother ever gave birth to such a huge thing."

The young tarsier is said to be born at a precocial, or relatively mature, stage, with much of its brain development complete. Most animals that gestate for a long time are precocial. In the elephant, for example, pregnancy lasts 21 1/2 months, said John Lehnhardt, assistant curator of mammals at the National Zoological Park. When the calf emerges from the womb, the mother gives it a kick, shaking the mucus from its trunk, and within minutes the animal is on its feet, ready to move. If it were helpless at birth, it would immediately fall prey to predators.