A 28-year-old primigravida at 24 weeks of gestation presents for antenatal review. On examination, you note a dark vertical pigmented line running from her xiphoid process through the umbilicus to the pubic symphysis. The structure marked **A** in the diagram represents this finding. Which of the following best explains the mechanism underlying this physiologic skin change?

Explanation

## Why "Increased melanocyte-stimulating hormone and estrogen-induced melanogenesis in pregnancy" is right The linea nigra is the darkening of the linea alba (a connective tissue band running from the xiphoid through the umbilicus to the pubic symphysis) during pregnancy. This occurs due to increased melanocyte-stimulating hormone (MSH) and estrogen-induced melanogenesis in pregnancy, leading to diffuse skin hyperpigmentation, especially in genetically predisposed areas such as the linea alba. This is a well-established physiologic change of pregnancy and is the direct mechanism tested in this question. [Williams Obstetrics 26e] ## Why each distractor is wrong - **Increased thyroid hormone production leading to generalized skin darkening**: While thyroid changes occur in pregnancy, thyroid hormone does not directly drive melanogenesis. The primary mechanism is MSH and estrogen, not thyroid hormone. - **Increased cortisol levels causing suppression of melanin inhibitors**: Cortisol actually suppresses melanogenesis and would not explain hyperpigmentation. The mechanism is active melanin production, not suppression of inhibitors. - **Increased progesterone-mediated activation of dermal fibroblasts**: Progesterone does not directly drive melanogenesis. Fibroblast activation is relevant to striae gravidarum (stretch marks), not linea nigra pigmentation. **High-Yield:** Linea nigra = darkening of linea alba in pregnancy due to MSH + estrogen-driven melanogenesis; physiologic, benign, and often fades postpartum. [cite: Williams Obstetrics 26e]