3D imaging of fetal head molding and brain shape changes during labor | Hacker News Reader

3D imaging of fetal head molding and brain shape changes during labor

journals.plos.org

37 pointsokket20 comments

Hats off to the women who agreed to give birth inside a MRI for this study. Wow!

One of the evolutionary theses about why our brains are at the size they are, and no larger / smaller is that we’ve hit the hard limit on what can exit a female body without killing either the parent or the baby. This paper adds a lot of credence to that theory.

Absent selective pressure for wider-hipped women.

This might explain why we see a significant number of birth deformation disabilities in East Africa. Recent migrations (past 0.5k-3k years) might have an impact. For example, it is thought that some parts of East Africa were originally occupied by a pygmy population. They inter married with the migrating Bantu (typically 1.5-1.7m) who also inter married with the Nilotes (Very tall. Masai, Samburu, Luo...). So you now have a descendant population with large variances in height, waist size and so on.

Size incompatibility is thought to have killed neanderthal women who paired up with non-neanderthal men.

Sounds plausible. I have an extended family member who was born after a multi-day labor [in the 40s] where the doctor asked of the father “do you want the baby or your wife to go home, because they’re not both going to make it”. He picked his wife; his wife picked the baby; both went home, but my aunt had life-long deficits from the multi-day traumatic labor and a forceps extraction.

thank god they picked both

Our brain size has been getting smaller in last 20k years, accelerating in the last 10.

10k you mean? Although it sometimes feels like people have smaller brains just lately, too...

It’s plausible that early humans broke into the “success through smarts” niche by a sudden expanded brain mutation that left them with a horrendous birth mortality rate, and that since then selection for mutations for smaller heads (and thus better birth safety) has been very strong (though hopefully not as strong as overall selection for intelligence- essentially evolution finding ways to pack more into a smaller space.)

Those huge Neanderthal brains were just the clunky and unstable Minimum Viable Product; ours are the more refined version.

> This paper adds a lot of credence to that theory.

This is a nice, high-resolution illustration of a phenomenon that has been known for a very long time. This data may inform an analysis of the detailed anatomical mechanics, but it doesn't tells us anything about the evolutionary theory that wasn't already known.

Yes, that is true, this is well known and fairly established. I was using ‘adds a lot of credence’ in a colloquial sense.

If I'm reading this right...

Skull and brain is crushed during natural birth, but usually recovers its normal size afterwards.

I wonder if 'lack of crushing' can be bad for C-section births?

> I wonder if 'lack of crushing' can be bad for C-section births?

I don't think they mentioned anything about anything in the "pro" for the deformation - in fact "These MRI findings suggest that the fetus is subjected to greater stress than previously thought, which could explain the high incidence of asymptomatic brain hemorrhages and retinal hemorrhages found after normal vaginal delivery."

This is something we've adapted to deal with, it's not an ideal event.

Thats what I mean - there might be benefits too. For example, a bit of crushing might trigger damage-response in the brain which leads to more growth.

Since this has been the case for millions of years, evolution might well use what appears to be a disadvantage as an advantage in some cases. Just like there are some kinds of tree which grow better with occasional forest fires.

There may be, but we'll need evidence for that. Right now we just see the trauma. Not everything that nature does is a benefit - evolution is a pragmatist. Our bipedal nature and need for a big brain requires certain sacrifices to be made. Our ability to make our skulls deform enough to give birth is the only evolutionary advantage it needs to give us to justify it - it's hard to survive as a species if you can't give birth. I wouldn't make a jump to "cesareans are bad" just because it may be right, especially without any evidence from this evidence being for that, and the evidence against being "brain trauma".

My medical friends say that csection births can have complications with fluids not draining fully from the respiratory system and sinuses etc leading to infections. This is one benefit of the squishing during natural birth to help squeeze out fluids where it is no longer needed. Sorry I don’t have anything to cite.

edit: misspelled “birth”

Not really - we've just settled into an equilibrium where our heads are as large as possible with an ""acceptable"" death rate of maternal and child death and injury from the process.

(I'm reminded of one of the plot items in the Vorkosigan saga, the great feminist space opera series, which has decantable artificial wombs - the ultimate labour-saving invention.)

So nobody told me about this but when my first was born his head was a cone. Like it was really really squished and tall. I was positive he was severely deformed and with all the courage I could muster, calmly asked, "first time dad question. What can you tell me about the shape of his head?" The nurse chuckled and assured me it was normal and would get round in hours.

Tl;Dr: yes they can get very squished.

That's bizarre! A lot of my friends coincendtslly had babies around the same time. The C-section babies were adorable with perfectly round heads, while the vaginals looked like Kanamits (obscure Twilight Zone reference).

Of course, all the strange-shaped heads morphed back to normal after a few weeks.

(What was particularly odd was one friend whose vaginal baby's head was, like a C-section baby, perfectly spherical... a few crude jokes were made regarding the implications...)

That's to be expected. It's how births are. When babies pass through the mother's birth canal, the tight fit temporarily squashes their heads, elongating their flexible skulls and changing the shape of their brains. The child with a round head either went through a wider birth canal or was smaller. Joking about a woman's genitalia postpartum is simply distasteful.

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