Hi
I breed chihuahuas and the soft spot is called the molera. It is very common for a chi to have an open fontanel, which is another name for it. Below, from the Chihuahua Club of America Handbook:
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Characteristics that are common to Chihuahuas:
( This explanation of the molera in Chihuahuas was taken from the Chihuahua Club of America handbook)
THE MOLERA
The Chihuahua as developed in Mexico and the United States has displayed a "soft spot" on the top of the head. In the Chihuahua this spot, or fontanel, is known as a MOLERA; and is the same as that found in human babies. In the past, this molera was accepted as a mark of purity in the breed, and it is still mentioned in most Chihuahua breed standards the world over.
It is important to note that while many Chihuahua puppies are born without the molera, there are probably just as many with one and its presence is nothing to become alarmed over.
As shown in the illustration below, the molera in a Chihuahua will occur on the top of the head and may vary in shape and size when present.
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I hope this helps you and welcome to the forums. This link will provide some info on other health issues if you are interested...
http://www.diamondchihuahuas.com/health.htm
Karen