Conventional wisdom says that umbilical hernias are inheritable, and a
dog with a hernia should not be bred. Conventional wisdom, however, does
not differentiate between "true hernias" where there is a defect in the
body wall, and "delayed closures", where a small bit of omentum slips
out of the area before the umbilicus closes. In 26 years of dealing with
breeders and reproduction, I do not have any evidence that "delayed
closures" are inheritable.

"True hernias" are indeed highly inheritable. They generally have a
thick cartilage edge with an irregular, more or less circular shape.
They may extend up into the diaphragm, causing a communication between
the thoracic (chest) cavity and the abdominal cavity. These are very
difficult to repair surgically because there is a great deal of tissue
missing and the tissue is hard and inflexible. They often require the
use of a mesh implant to close the defect. These hernias can be related
to other midline defects such as heart abnormalities and cleft palates.

"Delayed closures" are just that. 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. 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. Oxygen is
obtained through the umbilical cord from the placenta, where the
mother's blood stream exchanges oxygen and carbon dioxide with the
baby's circulation. When there are delayed closures in the heart, it may
be possible to hear a murmur at 4 or 5 weeks that is no longer audible
by 6 or 8 weeks. This is normal development. These holes, like the
umbilicus, must be present in the fetus and close over a period of time
after birth.

Omentum is a kind of slippery thin sheet of tissue which is present in
the abdomen. It provides several services to the abdominal organs. It
has blood vessels travelling through it to the intestine. Its surfaces
produce serous fluid which lubricates the abdominal organs and makes it
possible for them to slide against each other without rubbing. It
carries lymph nodes for the abdomen, and is a major depot for fat
storage. It partially contains the abdominal organs - especially the
small intestine - and supports them in a kind of "plastic bag". 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.

It is essential with either condition to "reduce" the tissue escaping at
least once daily. Turn the puppy on its back and gently massage the
protrusion and slide the contents back up into the abdomen. This lowers
the risk of a loop of intestine becoming strangulated in the protruding
tissue. 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. This
is viewed by most people as a hernia, and a serious problem. If the dog
is a year old, has a small bubble, or 'belly button', and it cannot be
reduced, has been there since it was a baby, and the dog is healthy and
well, it is pretty obvious that the bubble does not contain any
intestine. While the puppy is younger, it may not be clear. 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. NOTE: 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.

"True" umbilical hernias can make no progress in closing, ever. They
must always be surgically repaired. These individuals should never be
bred. These are quite serious.

I have never seen a bitch with a delayed closure and trapped omentum
bubble show any sign of problem in carrying a litter or whelping. I have
never seen one of these bitches produce puppies with hernias.

The catch is, you must know which you have. Most veterinarians don't
draw these kinds of conclusions or try to differentiate the two
different conditions. Your vet may or may not be of help to you in this.
Breeders with years of experience often know the difference intuitively.
Indeed, since "true" hernias are relatively rare, most have never seen
one, unless they are breeding a line that produces them. In these cases,
they see true hernias with great enough frequency that any breeder with
any common sense will abandon that line.