I looked it up. This is part of what it says about it. You'll want to read the whole article at the link I posted:
Depending on the size of the opening, abdominal structures such as falciform fat or omentum can float into the opening. This generally does not cause a problem. However, if the opening is large enough, an intestinal loop can become trapped which can become a life-threatening problem. For this reason, it is recommended that larger hernias be closed after diagnosis. This is most often done concurrently with the spay or castration surgery.
Umbilical Hernia in Dogs