When oxygen is transported from the mother to the foetus via the umbilical cord, a greater number of red blood cells are needed compared to when oxygen is transported from the lungs in a newborn baby. After birth, the turnover of haemoglobin in a newborn baby is high as the excess blood cells will be broken down. When haemoglobin is broken down, unconjugated bilirubin is formed that is taken up by the liver. A newborn baby's liver is immature and does not have the ability to process all the unconjugated bilirubin that is formed, which causes the proportion of unconjugated bilirubin in the blood to rise (physiological icterus) leading to yellowing of the skin and the whites of the eyes.