McCallum W D, McAreavey R M
Gynecol Invest. 1976;7(4):201-12. doi: 10.1159/000301383.
Human umbilical vein and artery diameters reduce significantly during the first 2 min after delivery, whereas blood gas tensions do not change significantly. Gas tension changes, therefore, cannot account for the reduction in the umbilical vessel diameters which occurs during this time.