The presence of iron causes brown menstrual blood at the beginning and end of a period when the flow is lighter. It is always advisable to consult with your doctor if you notice sudden changes in your menstrual discharge.
The uterine lining thickens throughout the menstrual cycle to protect a new pregnancy and sheds itself at the end of the cycle in the absence of a pregnancy. According to WebMD, menstrual blood is usually bright red when flow is at its heaviest, but it is often brown at the beginning and end of a period when flow is lighter. It takes longer for menstrual blood to be discharged from the body when flow is light, and the iron component of blood affects its color during this time. This brown blood is also known as old blood.