Duration of Labor

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Human Uniqueness Compared to "Great Apes": 
Absolute Difference
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Great apes typically give birth at night, and although complications can occur, the delivery often occurs over a period of a few hours or less, without assistance from group members, with the mother taking care of the process. Videotaped studies of great apes during labor have suggested that they do not grimace a great deal and do not vocalize in pain unless there is a complication of some kind. In striking contrast human labor can go on for many hours or even days, especially with the first child, and typically requires assistance from group members in order for a successful outcome. The reasons for this difference in the duration of labor is not clear.  There is of course the issue of the head being too large to exit the narrow human female pelvic outlet, and one possibility is that human labor simply takes much longer because of that difficulty. The converse possibility is that human labor is deliberately slowed down to prevent damage to the mother and the baby during the difficult passage through the blood canal. It is interesting to note that the uniquely human expression of Siglec-6 in the placenta appears to increase during the course of labor and reaches a maximum at the time of full labor, with very low levels typically found in placentas obtained following an elective cesarean section. Also Siglec-6 binds leptin, a molecule that is involved in the birth process.  However, there is no direct mechanistic connection between these findings and the duration of labor in humans.

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