The Chihuahua (/tʃɪˈwɑːwə, -wɑː, -ˈwaʊ.ə/ (listen); Spanish: chihuahueño) is the smallest breed of dog, and is named after the Mexican state of Chihuahua. Both British and American breed standards state that a Chihuahua must not weigh more than 5.9 lb (2.7 kg) for conformation. Both British and American breed standards state that a Chihuahua must not weigh more than 5.9 lb (2.7 kg) for conformation. This does not mean that they are not purebred Chihuahuas; they just do not meet the requirements to enter a conformation show. This is not a defect; it is a normal adaptation facilitating the passage through the birth canal and growth and development of the domed type of forehead. The molera is predominant in the apple heads and is present in nearly all Chihuahua puppies. The molera fills in with age, but great care needs to be taken during the first six months until the skull is fully formed.