Why 40 Weeks?

New research supports current guidelines for a full-term pregnancy.


Why are babies born at around 40 weeks of gestation? Experts have long suggested that moms must pop by then, otherwise, the baby's head would grow too large to traverse the birth canal.

But new research suggests a more sophisticated reason: Babies are born just before the mother enters a "metabolic danger zone." University of Rhode Island anthropologist Holly Dunsworth, Ph.D., challenged the idea that pelvic size triggers birth by comparing human gestation to that of other primates: Given our body size, human pregnancy is a bit longer than expected, and human babies a bit larger.

Rather than hip width determining the timing of childbirth, Dunsworth says, it's more likely that babies are born at nine months because a human mother just can't put any more energy into pregnancy, having reached the physiological maximum for burning calories. The 40-week limit makes babies far more helpless than other primates, the study notes.

A chimpanzee crawls at about 1 month of age, compared to about 7 months for a human baby. For a human newborn to be as developed as a chimp newborn, a woman would need to stay pregnant for about 16 months.

Read More: Pregnancy Timeline: Your Week-by-Week Pregnancy