Birthmarks are irregularities in the skin. Most birthmarks are present since birth and are usually benign. They can be present any part of the body. Some over age get darkened or become light or may completely disappear. The birthmarks that you have described looks more like a pigmented variety most likely being Vitiligo. This is a condition in which the melanocytes that are the cells that produce melanin are destroyed. It usually presents as whitish patches on the skin. The most common areas include hands, lips, feet, and arms. The exact cause is not known. It could be due to self-destruction of the melanocytes, autoimmune disease, stress, and sunburn, hereditary.

Vitiligo appears in the form of three patterns:

a) Focal pattern: where the patches are limited to a limited area.

b) Segemental pattern: it develops on one side of the body

c) Generalised pattern: occurs on both sides of the body.

Usually Vitiligo does not spread. However in certain individuals, there are chances of spreading.

I would suggest you to get a biopsy of skin to confirm the diagnosis and get a complete blood test including TSH levels, antinuclear antibodies. Usually there is nothing to worry , however if you want to get it treated you can use a topical steroidal cream which enhances the pigmentation for three months, phototherapy , and surgery like skin grafting .Hope this helps.