What it is
A puppy is born with a skull made of separate plates of bone, joined by flexible seams so the head can grow. On the very top, where several plates meet, there is a soft gap: the fontanelle. As the puppy grows, bone gradually fills it in. Sometimes it closes only partly, or not at all, and a soft spot remains.
An open fontanelle is linked to small, toy-sized dogs with a rounded, domed head. On its own it may cause no trouble, but it can go along with a build-up of fluid around the brain, so a vet should check what lies beneath it.
In everyday life, the soft spot calls for a gentle hand. Stroke the head from the sides and the back of the skull rather than pressing on top, keep your puppy away from rough play with bigger dogs, and use low furniture or steps by the sofa so there are fewer jumps and falls.
Signs you may notice
A soft spot may be visible or noticeable on top of the puppy's head where the skull bones have not joined. If you notice such a spot, it is worth having the puppy seen by a vet.
When to see a vet
Book a visit if you feel a soft spot on your puppy's head. A seizure, a blow to the head or sudden dullness means a vet is needed immediately.
This is general information, not veterinary advice. For your own dog's health, tests and treatment, see a vet.