Umbilical circulation--physiology and pathology.

1. The umbilical circulation under physiological conditions is protected by the amniotic fluid. This protective mechanism of the amniotic fluid may be disturbed artificially or spontaneously by the rupture of the membranes. 2. Umbilical cord compression is of no harm to the fetus when it is mild, since the fetus possesses a circulatory buffer system when umbilical blood flow is in a physiological range. However, severe reduction of umbilical blood flow may lead to fetal hypoxia. 3. Chronic or repetitive acute fetal hypoxia leads to deterioration of fetal circulation and umbilical blood flow. This can be recognized by the obstetrician during labor based on fetal heart rate patterns.