"This happens because of a phenomenon known as the "fight or flight response..."

This comes from the work of Grantly Dick-Read, the "father" of "natural" childbirth. Ironically, most homebirth advocates don't realize that Grantly Dick-Read made it all up. What's even worse is that he made it up because he was a racist and sexist who believed that women should not be educated or have careers and instead should devote themselves to having more children than their racial "inferiors".

In Holistic obstetrics: the origins of "natural childbirth" in Britain, Dr. Ornella Moscucci writes:

"Health policy became the subject of intense public debate in the aftermath of the Boer war ... Adherents to the new science of eugenics ... blamed heredity. In their view, health policy should aim to prevent reproduction among "low quality" human stock .., and encourage reproduction among "good" stock...

One obvious way to reverse the falling birth was to entice women of "superior stock" back into the home, where they would fulfil their functions as wives and mothers. Health reformers took up the challenge by developing an ideology of childbirth that emphasised the "naturalness" of pregnancy and birth..."

"The argument developed [by Dick-Read] mixed Darwinian themes, neurophysiological theories, and cultural stereotypes of childbirth among "primitive" people... Whether women experienced pain or not depended on cultural attitudes to childbirth rather than on some property inherent to parturition. Dick-Read ... claimed that primitives experienced easy, painless labours. This was because in primitive societies the survival value of childbirth was fully appreciated and labour was regarded as nothing more than "hard work" in the struggle for existence. In civilised societies on the other hand a number of cultural factors conspired to distort woman’s natural capacity for painless birth, producing in woman a fear of childbirth that hindered normal parturition..."

So Grantly Dick-Read, who is estimated to have witnessed perhaps one or two births in his career, made it all up. And he made it all up because he was afraid of women's emancipation.

In the article,The Race of Hysteria: "Overcivilization" and the "Savage" Woman in Late Nineteenth-Century Obstetrics and Gynecology, Laura Briggs argues that the comparisons between "overcivilized" white women and "primitive" women who gave birth easily was not merely racism, but reflected men's anxiety about women's emancipation... Simply put, the result of women insisting on increased education, roles outside the home and political participation was that their ovaries shriveled, they suddenly began to experience painful childbirth and they developed the brand new disease of "hysteria", located in the uterus itself.

Briggs writes:

Hysteria ... was never just a disease. It was also the way nineteenth century U.S. and European cultures made sense of women’s changing roles... [C]ultural changes were accompanied by a virtual epidemic of “nervous weakness” ... nervousness was often characterized as an illness caused by "overcivilization," which ... defined cultural evolution as beginning with the "savage," culminating in the "civilized," but also containing the possibility of degeneration — "overcivilization." ... As a disease of "overcivilization," hysterical illness was the provenance almost exclusively of Anglo-American, native-born whites, specifically, white women of a certain class..."

Furthermore:

... "[O]vercivilized" women avoided sex and were unwilling or incapable of bearing many (or any) children, "savage" women gave birth easily and often, and were hypersexual... [B]y insisting that white women were becoming sterile and weak while non-white women remained fertile and strong, it encoded white women’s transgressive behavior as a danger to the future of "the race..."

In Brigg's analysis, pain in childbirth served a very important function in this racist and sexist discourse: it was the punishment that befell women who became too educated, too independent and left the home. The idea that "primitive" women had painless childbirth was fabricated to contrast with the painful childbirth of "overcivilized" women.

When Grantly Dick-Read and his peers claimed that "primitive" women had painless labors, they were issuing a warning to women of a certain social class: if you dare to step beyond the roles that we have prescribed for women, you will be punished with painful labor. And if you have had painful labor, you should understand it as a punishment for ignoring your "natural" duty to stay home and procreate. In light of this, it is more than ironic that some contemporary women are still insisting that childbirth is not inherently painful, that indigenous women have painless childbirth, and that if you "prepare" for childbirth properly, your birth will be painless, too.