The umbilical cord extends from the umbilicus of the foetus to the surface of the placenta. It carries the foetal blood from the foetus to the placenta via the two umbilical arteries, and the returning blood via the single umbilical vein.
The blood vessels in the umbilical cord have a very thin muscular wall and are easily damaged. They are surrounded by a jelly-like substance known as Wharton’s jelly and are coiled in a spiral fashion. The umbilical cord is approximately 50 cm. in length and in a fully mature pregnancy is about the thickness of the index finger. It is surrounded by a very thin, delicate, skin-like membrane.
On rare occasions the umbilical cord becomes tied into a true knot which may be pulled tight either by the foetus within the uterus, or at the time of delivery. A true knot which is loosely tied does not affect the circulation of blood, but if it becomes tightly pulled then the blood flow will be obstructed and the foetus will rapidly die from lack of oxygen. This type of knot in the cord is rare, but is an unpreventable cause of death. In early pregnancy the baby can move freely within the uterus and undoubtedly turns over and over many times. It is extraordinary that true knots do not form in the cord more frequently.
The umbilical cord may become wrapped around an arm or a leg, or it may become twisted around the neck of the baby. Instances have been recorded in which the umbilical cord has been coiled as many as six or seven times around a baby’s neck and the child still survived unharmed. Nevertheless, this does present a hazard to the baby, mainly during labour. The placenta is usually attached to the top or the fundus of the uterus and as the baby descends during the second stage of labour so the cord which is twisted round the neck tends to be pulled tighter with each’ descending movement. The cord is usually sufficiently long to allow this to happen without any effect on its circulation, but if it is rather short then the circulation will gradually be occluded as the head of the baby descends and the cord is tightened round the neck. Happily this catastrophe is extremely rare, but unfortunately it is impossible to diagnose before delivery or before sudden and dramatic foetal distress has occurred during the second stage of labour.