What is the meaning of the dark (black) median line that always appears on the abdomen of pregnant women?

The dark (black) median line that appears on the abdomen of pregnant women is called the linea nigra. It is a common occurrence during pregnancy and is caused by hormonal changes in the body. The linea nigra typically appears in the second trimester and runs vertically from the navel to the pubic bone. Its purpose is not fully understood, but it is believed to be related to increased pigmentation due to hormonal fluctuations. The linea nigra usually fades or disappears after childbirth.