The umbilical cord is a pretty amazing structure. In the developing fetus, it goes from the placenta (the afterbirth) of the mother, through the baby's navel and into the fetal liver and heart. This is how oxygen and nutrients are conveyed into the growing fetus.

If the baby's blood pressure gets too high, small blood vessles around the umbilical cord expand and cause the abdominal wall to spread. This then allows two arteries that run from the fetal aorta by way of the bladder to conduct excess blood back to the placenta. All of these blood vessels are usually still open at the time of birth and tubes may be inserted into them to reach the circulatory system or the heart for measuring blood pressure or obtaining blood samples.

Another passage leaves the bladder and goes out the navel, allowing the developing fetus to expel urine. This eventually shrivels into a ligament connecting the bladder to the abdominal wall after birth. In some cases, this passage may still be open at the time of birth, allowing the baby to expel urine through an open navel.

Even in adults, the lumen of the umbilical vein does not completely disappear and sometimes, it may be opened in order to inject chemotherapeutic drugs.