There is an area in the abdominal wall where the great vessels of the umbilical cord exit the body of the fetus to derive nutrition and oxygen from the placenta. After birth, these vessels close and shrink up. There is left behind a small area in the midline where the vessels formerly escaped the body. The nature of this structure is a linear slit in the midline, lined with normal connective tissue. The hole, like the umbilicus, must be present in the fetus and close over a period of time after birth.

A "True" umbilical hernias can make no progress in closing, ever. When the hole want to close but it is not possible because omentum is in the way we call it delayed closure.
Omentum is a kind of slippery thin sheet of tissue which is present in the abdomen. It provides several services to the abdominal organs. As the puppy puts on weight, it is possible for a bit of this thin slippery tissue to protrude through the umbilical slit. As the normal process of closing of the umbilicus proceeds, it is possible for this bit of tissue to be entrapped. The danger here is whether it is solely omentum that is escaping, or if the defect is large enough for a loop of small intestine to escape the abdomen as well.

If the 'hernia' is a closure defect, the normal process of closing will continue, and at some point a small amount of omentum may be entrapped in a bubble outside the essentially closed body wall. The very tiny holes with a small bubble of omentum do not require surgery. Slightly larger holes should be closed to be sure that no intestine becomes strangulated in the process of closure. No delayed closure can ever close completely, simply because there is something sticking through it. If there were no omentum sliding out, they would continue to close normally. What they do, is they close tightly around the omentum, trapping it outside in an absolutely firm unchanging bubble. So you see there is a different between a umbilical hernia and a delayed closure.

Dogs witch a "True" umbilical hernias can make no progress in closing, ever, the hole will never get smaller, it can only become bigger in time. They must always be surgically repaired. These individuals should never be bred. When a true umbilical hernia exist it is possible that other structures in the body does not close. There are other structures on the 'midline' that undergo similar development after birth. In the heart, there is a hole between the upper chambers that allows the fetus to bypass the lungs, which, of course, cannot contribute any oxygen to the blood before birth. When this hole does not make progress in closing, it is possible to hear a murmur. The normal development is that this hole close at 4 or 5 weeks, even sometimes after 7 weeks. When a breeder take the puppy's with six weeks to the vet, they should hear if a murmur is present or not any more. When it is still present more examination is acquired.