"If It Waited for a Full Term, It Would Tear Its Mother in Half"?

May 1, 2011 8:57 AM

When I read Cory Doctorow's "I, Row-Boat", a character (Olivaw) says that human beings "have to be born half-gestated because its head would be so big if it waited for a full term, it would tear its mother in half." I had not heard of this viewpoint before, and am wondering where Doctorow heard of this idea. I tried sending him an e-mail, but received no response.
posted by WCityMike to science & nature (10 answers total)

I've read this all over the place; here it is at blogs.discovermagazine.com -- The ultimate and proximate reason for this relative underdevelopment of human newborns is usually attributed to our huge brains, which run up against the limiting factor of the pelvic opening of women. If a human baby developed for much longer through extended gestation then the mortality rates of their mothers during childbirth would rise.
posted by kmennie at 8:59 AM on May 1, 2011 [1 favorite]

Also search for the term "The Forth Trimester"
posted by bottlebrushtree at 9:04 AM on May 1, 2011

I don't have any helpful links for you, but in my biological anthropology/ evolutionary biology classes in college, this was something we discussed frequently. The evolution of the human brain has resulted in a constant struggle between the size of babies' heads and the width of their mothers' pelvises. Two of the compromises that have resulted from this struggle is that women have slightly wider pelvises than men (a trade-off because it makes us less efficient bipeds) and babies are born earlier or rather, with less development than they otherwise would, giving their heads the chance to grow more outside of the womb, but making them more helpless than the newborns of other species.
posted by DeusExMegana at 9:11 AM on May 1, 2011

Fascinating! Thanks for the links, guys. Last time I tried to Google-search for this, I evidently couldn't find the right keywords to turn up anything.
posted by WCityMike at 9:18 AM on May 1, 2011

The compromise between bipedalism and the size of the human skull also necessitates that the child rotate as it is born. The birth process is simpler for other mammals and primates.
posted by jedicus at 9:20 AM on May 1, 2011 [2 favorites]

Um, you probably already figured it out, but it's The Fourth Trimester, not the "forth."
posted by BlahLaLa at 9:26 AM on May 1, 2011

If you're interested in reading more about this, the book Our Babies, Ourselves by Meredith Small has a chapter about the biological and evolutionary forces at work in this. It traces human evolution back and discusses the changes in pelvic shape and the accompanying adjustments to the gestation period. Fascinating book.
posted by MeghanC at 10:34 AM on May 1, 2011

I think I've read about this in books by Robert Sapolsky or Stephen Jay Gould. Both highly recommended, in any case.
posted by theora55 at 11:08 AM on May 1, 2011

Human birth and bipedalism. With models.
Brain development and the female pelvis.
Evolution of human birth.

No idea where Doctorow heard of the idea, but it is very well-known in medical/evolutionary/anthropological circles.
posted by gingerbeer at 2:22 PM on May 1, 2011

FWIW, I'm pretty sure it's also mentioned as an aside in one of Asimov's Robot novels (if not one of the robot novels, then one of the short stories or the Foundation novels). That'd fit in with the rest of the conceit Doctorow uses in that story.

The exact quote escapes me, but it's something like "it's the size of the human head that makes childbirth a such a dangerous and difficult process"
posted by Pinback at 3:23 PM on May 1, 2011