Why is my 4-month-old puppy's tummy turning black and spreading?

This looks like hyperpigmentation. It typically occurs in lightly pigmented, sparsely haired regions as dogs get older. It can also be caused my inflammation so just make sure she is not spending too much time sunbathing and that there are no other signs like licking/scratching/lesions. I hope this helps and thank you for using Petco Pet Education Center, formerly Petcoach!

Without images or a physical exam, it can be difficult to determine definitively what is on your pup's skin. That said, many puppies have patches of [darker skin](https://www.petcoach.co/dog/condition/pigment-changes-of-the-skin-and-coat/) on their bellies. If these areas are flat, smooth to the touch and not bothering him, then it is probably just his skin coloration. If the areas are itchy or raised, then it would be a good idea to have them checked out by a vet since this may indicate a [skin infection](https://www.petcoach.co/dog/condition/staphylococcal-pyoderma-1/) or some other problem.

The pictures look like it could just be normal pigment change. As puppies mature their pigmentation can as well. If the skin is raised, thicker, crusty, or itchy then I would be concerned. If you have any concerns then I would encourage you to have him evaluated by your veterinarian. If there is a concern then a biopsy could easily be done at the time of his neuter. Best of luck and thank you for using Pet Coach.

Thank you for asking the question about Harley. If she is not itchy then it may just be pigment change to her skin. Lots of breeds can have changes to their skin and Hair coat as they mature. If you happen to have before and after pictures to post in another question that may be helpful. Is she is itchy, or the skin has wounds or crusting then she should be evaluated by a veterinarian. Best of luck and thank you for using Pet Coach.

Hyperpigmentation of the skin is common with age where there is no overlying hair. It can also be caused by skin irritation/infections but if there are no lesions or symptoms it is likely normal. No treatment needed in that case. I hope this helps and thank you for using Petco Pet Education Center, formerly Petcoach!

It looks like it may the start of puppy pyoderm and hers it no severe. Usually it is something they outgrow! Sometimes it can spread and if you see small pustules that will look like zits then she will need medicine. otherwise, keep the area clean with bath once a week and just watch it.

If the skin is not irritated/itchy and feels the same as the skin surrounding that is not dark in color, I would just monitor the area. We can have changes in pigmentation of the skin in dogs as they age, as well as changes as the result of inflammation. If the area begins to spread, or the skin feels dry/rough in this area, or she is licking, a visit to the vet is recommended.