What hormone increase is primarily responsible for hyperpigmentation during pregnancy?

During pregnancy, the primary hormone responsible for hyperpigmentation is estrogen. Increased levels of estrogen contribute to the stimulation of melanocytes, the cells that produce melanin, leading to darker skin pigmentation in certain areas of the body. This phenomenon, sometimes referred to as the "mask of pregnancy" or chloasma, is often most prominent on the face but can also affect other regions.

While progesterone, human chorionic gonadotropin (hCG), and relaxin play significant roles during pregnancy, they are not primarily linked to the increase in melanin production responsible for hyperpigmentation. Progesterone is mostly involved in maintaining pregnancy and regulating various aspects of the menstrual cycle. hCG is crucial for early pregnancy maintenance and supporting the corpus luteum but does not directly influence skin pigmentation. Relaxin primarily aids in softening the cervix and pelvic ligaments to prepare for childbirth. Thus, among the options provided, estrogen is the hormone directly responsible for the hyperpigmentation observed during pregnancy.