Since this intricate system permits the fetus to obtain oxygenated blood and nutrients from the placenta, I’m sometimes curious about how the circulatory system works for fetal compared to adult. I know it’s made up of the placental blood vessels and the umbilical cord, which has two umbilical arteries and one umbilical vein. However, I discovered in this book that there are significant individual differences and that the pattern changes with gestational age. The normalized umbilical blood flow decreases with gestational age, and a new stage of development appears to be attained at 28 to 32 weeks. Increased ductus venosus and foramen ovale shunting, increased impedance in the lungs, reduced impedance in the brain, increasingly reversed flow in the aortic isthmus, and more prominent coronary blood flow are all responses seen in the human fetus in response to increased afterload, hypoxaemia, and acidaemia.

The fetal circulation – PubMed

https://pubmed.ncbi.nlm.nih.gov/15614842/

Accumulating data on the human fetal circulation shows the similarity to the experimental animal physiology, but with important differences. The human fetus seems to circulate less blood through the placenta, shunt less through the ductus venosus and foramen ovale, but direct more blood through the …