Final answer:
Blotchy hyperpigmentation on a woman's forehead during pregnancy is likely caused by hormonal changes, specifically chloasma, which is due to increased melanin production stimulated by hormones like estrogen and melanocyte-stimulating hormone.
Step-by-step explanation:
The reason behind the blotchy hyperpigmentation on a woman's forehead at 22 weeks of gestation is most likely due to hormonal changes associated with pregnancy. This condition, known as chloasma or the "mask of pregnancy," is characterized by blotchy brown patches of skin on the face. It's a common effect of elevated estrogen levels, which alongside an increase in melanocyte-stimulating hormone during pregnancy, can lead to increased melanin production and subsequent skin discoloration.
While there are other potential causes for changes in skin pigmentation, such as allergic reactions, lack of sun protection, or iron deficiency, these patches of skin darkening during pregnancy are most commonly related to the hormonal and physiological changes occurring in the body. It's crucial to understand that these skin changes are often temporary and typically resolve after childbirth.