Final answer:
Melasma is the pigmentation disorder that occurs only during pregnancy or while taking birth control pills, leading to blotchy brown patches of skin on the face.
Step-by-step explanation:
The type of pigmentation disorder that occurs only during pregnancy or while taking birth control pills is Melasma. This condition is also known as chloasma or the "mask of pregnancy." It leads to blotchy, brown patches of skin, mainly on the face, which result from increased melanin production stimulated by hormonal changes.
Melasma is different from other pigmentation disorders like albinism, which is a genetic condition where there is an absence of melanin throughout the body, or vitiligo, where there is a loss of color in patches due to the destruction of melanocytes.