According to this article:

"If a blood pressure gauge is placed on an unclamped umbilical cord, it will pick up pressure rises as high as 60 mm Hg with each uterine contraction. This indicates that these contractions are intimately involved in the transfer of placental blood through the cord. A striking pressure rise, which persists through the first few hours of life, is also evident in the baby's vena cava and right atrium of the heart. All studies on this indicate a sign...