Why is birth painful?

I came across an interesting article in Salon suggesting that the pain of childbirth basically serves no purpose and is merely a hangover from evolution. Some anthropologists have argued that labour pain served as a means of letting women know that they should seek assistance, ultimately, as a mechanism to lower rates of infant and maternal mortality. If women received help, they were less likely to bleed to death. The strongest evidence seems to be that all cultures have traditional birth attendants which suggests that assistance during birth was considered to be critical.

The question remains, it seems, as to whether labour pain has lived out its usefulness for women. As the author of the article suggests, this theory is problematic for natural childbirth advocates because this evolutionary perspective implicitly argues that birth is dangerous and not 'natural'.

I'm not sure I agree with this argument in general. After all, babies heads have become substantially larger over time and a woman's pelvis has pretty much stayed the same. In a nutshell, it seems pretty clear to me why giving birth would hurt.

Source: http://open.salon.com/content.php?cid=31854