Molera
The Chihuahua's molera(a.k.a. fontanel) is considered a breed characteristic and not a defect. Most Chihuahuas (80 percent to 90 percent) have a molera — a soft spot on the top of their head similar to a human baby's soft spot. But unlike babies, most Chihuahuas don't outgrow it. Although it usually shrinks as the dog matures and ends up between nickel- and dime-sized, Pepe's molera won't be a problem as long as you're gentle when petting or handling his head.

In rare cases, the molera remains quite large and can be a sign of a serious problem called hydrocephalus (see the next section). But don't borrow trouble. Hydrocephalus has several other signs besides a larger-than-usual molera.

Hydrocephalus
A dog with hydrocephalus (a.k.a. water on the brain) may have an unusually large head for his size caused by swelling. Other signs of this fatal condition are frequent falling, seizures, a lot of white showing in the eyes, an unsteady gait, and east-west eyes (the opposite of crossed eyes). A dog with hydrocephalus is in pain and won't live long, so euthanasia is the humane solution. (Euthanasia is the medical term for a humane, vet-assisted death.)