I’m thinking about that moment that s/he goes from being supported by a version of external gills to breathing air. That stretch from underwater being to air-breathing mammal. What is happening physiologically during that transition? Amounts of oxygen in the blood, or pulse rate, e.g. Does anyone know? Are there studies? How would one find out what they say?
There’s more than that going on. Passage through the mommy’s pelvis requires some distortion, especially of our oversized heads. There are models of this birth process at the American Museum of Natural History in NYC and at the Museum of Science in boston (and in circa 1960 copies of World Book Encyclopedia).
“EXternal gills”? Oxygen, like nutrients, comes through the blood through the umbilical cord from the placenta and, ultimately, from mommy. It’s a stretch to call that “gills”
There’s a fatal flaw in your thinking. Namely, babies don’t breath through any kind of gills before borth (IIRC, in the very early stages of developement, we do have gills in the womb, but we lose thm long before birth. Someone correct me if I’m wrong.)
All the babies O2 comes from the mother’s blood and passed to the child throguh the umbelical cord. As such, it doesn’t need to breath at all. Likewise, if we were to hook you up to a machine that ogygenated your blood and removed excess CO2 from it, you could hold your breath for a very long time. 
Yeah, I agree that the reference to gills seems a little misplaced, but the transition from being supported via the cord to using his/her own lungs for the first time is certainly a significant one. Never really thought of things that way before.
Have no hard data of the type you were lookin’ for though.
While in utero, the pressure on the inside of the lungs is equal to the outside, so the lungs are only a potential space. Fetal circulation is different than ours. There is an opening in the atria that allows venous and arterial blood to mix, and there is a small vessel that connects the pulmonary artery and the aorta, bypassing the lungs. the vessels from the umbilical cord circulate the fetal blood back to the placenta where oxygen and CO2 are exchanged.
When the fetus becomes a baby, by leaving the protection of the uterus, the pressure around the lungs becomes negative to the outside. When that happens, the lungs expand for the first time. Someone in attendance suctions the baby’s mouth and nose, so nothing obstructs its breathing.
Over the next few days, the fetal circulation closes* and the baby has circulation similar to everyone one else.
*Occasionally, it doesn’t close. When that happens, the doc will first prescribe medication to help it close, if that doesn’t work, it has to be closed surgically.
Did I answer you question?
I have heard that the squeeze of a vaginal birth does squeeze some fluid out of the baby’s lungs and that’s a con to c-sections.
Not true, vaginal birth does stimulate the babe, but there isn’t a significant amount of fluid in the lungs to be “squeezed” out. C-section Puts the mother at more risk than the baby.
There are no gills ever in the human. During development, the pharyngeal arches resemble gills and IIRC from zoology, in fish the gills develop from similar structures, but the gills are highly specialized organs and there is no equivalent in humans. The fetus swallows and basically urinates amniotic fluid, and while the lungs are fluid filled as others have pointed out they are not inflated.
The transitions of anatomy that happen with the first breath are truly incredible and have been very well studied, down to the molecular level. The story of the first breath is all about blood pressures and pressure differentials and hydrodynamics, followed in the next few days by molecular signaling by prostoglandins and other endothelial growth factors. The previous links to the fetal circulation kind of go over that, but the short of it is that all oxygenation and most detoxification in the fetus is taken care of by the mother. Oxygenated blood comes in through the umbilical vein and deoxygenated blood out through the two umbilical arteries.
Oxygenated blood courses up through the umbilical vein through the ductus venosus in the liver to the right side of the heart through the inferior vena cava, mixing with blood returning from the peripheral tissues. The right heart pumps to the lungs, and three separate shunts ensure that most of the output instead gets pushed to the left side of the heart and its circulation, pumping to the peripheral tissues. These are the foramen ovale, the ductus arteriosus, and the aforementioned ductus venosus. The umbilical arteries come off of the iliac arteries to return blood to the mother. There is a lot of oxygenated/deoxygenated admixture from all of this – only partially deoxygenated blood gets returned to the mother, and oxygenated blood from the mother is mixed at several points with venous blood.
With the first breath, the lungs expand, lowering resistance in the right side circuit. Obviously blood stops flowing through the umbilical veins and arteries. The foramen ovale shuts and the ductus arteriosus and venosus begin to close down. This means that the right side output is now mostly directed to the lungs, which return oxygenated blood to the left side of the heart, which now pumps to the tissue. Most admixture is stopped within the first few days with complete closure of the ductus and the foramen ovale.
Ahh, that must have been what I was thinking of. Honestly, the only time I really learned about that was some off hand comment by my ninth grade biology teacher. Odds are he said something to that effect, and my memory over the years turned it into “we had gills.” 
Ok, so my reference to gills was an over-the-top distorted metaphor. I understand that there are no gills, and never were, despite the “ontology recapitulates phyogeny” cannard that some of us learned. I merely meant to suggest that the oxygen was coming to the fetus by a mode that did not include his breathing with lungs. It made me think, for too brief of a moment obviously, that it was roughly analagous to the transition from tadpole to frog. I think Edwino and picunurse are providing a better picture of what is a truly mind-boggling transition. I had begun thinking about the topic by realizing that in addition to all the other changes of environment that the fetus experiences, birth also is the first time that he gets to actually stretch - a wonderful sensation for most of us, but what’s it like for the fetus? From there to wondering about this incredible transition. Again, aside from neonatology texts, where does one learn about these things? xo, C.
picunurse, there are lots of studies showing that babies born by c-section suffer higher rates of respiratory distress and pulmonary hypertension. Several reasons for this have been proposed, including anaesthetic and misjudged due dates leading to premature delivery. But the idea that the squeezing action has something to do with better lung function is also a valid hypothesis.
From Milner AD, Saunders RA, Hopkin IE
Arch Dis Child 1978 Jul;53(7):545-8:
From **Obstet Gynecol 2001 Mar;97(3):439-42
Levine EM, Ghai V, Barton JJ, Strom CM.
Department of Obstetrics & Gynecology, Illinois Masonic Medical Center, Chicago, Illinois 60657, USA. **