Years ago, when I was a doula, they used to tell women that they couldn't have their babies on their chests for exactly the opposite reason: if the baby was on the woman's chest, it would get too much blood from the placenta and that would be bad. I must say that this ob takes ignorance to a new level. We're all pretty used to obs who don't have their facts right, but here's one who can't even get the myth straight.
Jokes at the ob's expense aside, the shut down in blood flow through the umbilical cord isn't a passive process. The new edition of Obstetric Myths Versus Research Realities (University of Michigan Press), co-authored by Amy Romano and me, will have a chapter on newborn transition. Here is an excerpt from the manuscript for that chapter:
"At any given moment, a substantial proportion of fetal blood supply is circulating through the placenta. After the birth, the baby recaptures much of that supply. The umbilical arteries, which deliver deoxygenated blood from the baby to the placenta, constrict reflexively in response to rising oxygen levels, the drop in ambient temperature, and other stimuli, while the umbilical vein remains open for several minutes. This one-way street results in a net blood flow to the baby after birth that can amount to as much as 40% of the baby’s blood volume (Mercer 2002). As long as the placenta remains attached to the uterine wall, this blood is oxygenated, providing a safety cushion as the newborn transitions to air breathing. Even after placental detachment, deoxygenated blood may continue to flow and augment the baby’s blood volume as the contractions of third stage force additional blood out of the placenta (Mercer 2002)."
Mercer JS, Skovgaard RL. Neonatal transitional physiology: a new paradigm. J Perinat Neonatal Nurs 2002;15(4):56-75.
So you see, the backflow and shut down depends on an elegant feedback mechanism based on blood oxygenation, not where the baby is relative to the placenta. The real problem comes with early cord clamping, which cuts (pun intended) the process short, depriving the baby of a substantial proportion of its blood supply by trapping it in the placenta.
-- Henci |