If it weren't for the size limitations of a woman's pelvis, babies would stay developing in the womb for considerably longer, comparative biologists have suggested.

"We have to keep our pelvises relatively narrow to keep upright," said Lise Eliot, neuroscientist and author of What's Going on in There? How the Brain and Mind Develop in the First Five Years of Life (Bantam, 2000). To fit through mom's, er, escape hatch, the newborn brain is one-quarter the size of an adult's.

Accordingly, some pediatricians label a baby's first three months of life as the "fourth trimester" of pregnancy to emphasize how needy, and yet devoid of social skills, babies are at this stage. The first social smile, for example, doesn't usually appear until the infant is 10-14 weeks old and the first phase of attachment, scientists suggest, begins around five months old.

Some evolutionary biologists theorize that newborns are socially inept – and have an annoying cry – so that parents won't get too emotionally attached while the baby has an increased likelihood of dying. Of course, crying also gets a baby the attention he needs to survive.

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