During a menstrual cycle, the blood may become dark red or brown near the end of the cycle because the blood is older and is not being released as quickly. This is a normal color change during a menstrual cycle, explains WebMD.
The lining of a woman's uterus becomes thick during ovulation, elaborates WebMD. This is to prepare for a fertilized egg to implant itself in the uterus. When an egg is not fertilized, meaning a woman does not become pregnant, that uterine lining is expelled along with blood. This is the start of the menstrual cycle.
A normal period releases between 4 and 12 teaspoons of blood and other fluids, according to WebMD. The blood and fluids are released during a two-to-seven-day period though the average length of the menstrual cycle is between three and five days. The blood can start out as red and may have some clots at the beginning of the period or menstrual cycle. As the period continues, the blood can change color as it ages. The darker color in period blood is normal. When there are any sudden changes in a period from one month to another, periods are persistently heavy, or excessive clotting is noted, a physician should be consulted.