Labor is a very painful experience at least for the mother, and all of us can know this, if we aren´t mothers, by empathy (Varela, 2001). But, not just for the mother, Ferenczi (1983) talks about the birth`s trauma, previously described by Freud. He explained that because the baby lived in an very comfortable, secure, silent, warm and dark environment, the birth is an unpleasant experience for the first encounter with light and noise, a trauma that accompany us for the rest of our lives (Ferenczi, 1983). That can explain the baby`s pain and the mother`s pain, I`m inclined to say that is, as you said, for the size of head and vagina. So, if that´s the problem, an "adaptative" solution (in terms of darwinian hypothesis), would be bigger vaginas (¡). Can you imagine it?¡. Let`s remember that evolution doesn`t have a teleology, an aim (1), as our culture be prone to, I think that evolution is an spontaneous process (Maturana & Varela, 1990).
(1)As an example for the evolution as an historic phenomena with an aim, I`m thinking in Huxley in his "Evolution, the modern synthesis" (correct me if I`m wrong) with his evolutive progress in the continuous increase living being`s independence from the medium, with the human being crowning the process (Maturana & Varela, 1990).
Ferenczi,S.(1983) Thálassa, una teoría de la genitalidad, 4th Ed. (A. Mariam, Trad.): Letra Viva.
Maturana,H.,Varela,F.(1990) De Máquinas y Seres Vivos, auopoiesis de la organización de lo vivo, 2nd Ed.: Universitaria.
Varela,F.(2001) Interview in: http://www.youtube.com/watch?v=3-VydyPdhhg
, recovered on June 1st, 2012.