Menstrual cycle is the monthly cycle experienced by women as a result of the changes undergoing in the uterus and the vagina under the influence of female sex hormones. Menstrual cycles have lots of variations right from the age at which starts to the number of days it lasts to the color and the consistency of the menstrual blood.
The menstrual cycle consists of 4 phases which brings changes in the uterus based on the levels of hormones. First phase is the bleeding phase which lasts for the first five to six days. In this phase, the thick lining of the uterus supplied with tortuous blood vessels breaks down and sheds along with the blood. This phase is also commonly called as the weeping phase as there is weeping because of a missed pregnancy. This phase occurs if the ovum is not fertilized bya sperm and no pregnancy has occurred. Phase two is also known as the pre-ovulatory phase. In this phase the lining of the uterus slowly regains its normal thickness and then becomes more smooth and lubricated so as to prepare for the attachment of the fertilized ovum (female gamete) in the next phase. Phase three is the ovulatory phase. In this phase, the ovum is released from the ovary and travels to the uterus through the fallopian tubes to be fertilized by the sperm (male gamete). The ovum awaits union for two to three days else withers off and shrinks into a small mass called the corpus luteum. Phase four is the pre-menstrual phase. Since there no pregnancy has taken place, the hormone titers fall drastically and there is a cascade of changes taking place. The uterus lining which was prepared for the implantation of the fertilized ovum starts breaking down as the hormone levels dip leading to bleeding from the blood vessels and tissues of the lining. All this forms the content of the menstrual bleed.
Normally, the menstrual blood is dark red in color and in the fluid state. If the blood to be shed out as menstrual blood gets oxidized, it starts appearing dark brown in color. If the blood is oozing out but gets stored amongst the broken uterus lining for a considerable amount of time before actually coming out as of the vagina then it appears dark brown. Blood gets darker and darker as it gets old. The moment there is dark colored blood seen in menses it proves that the blood stayed a little longer before being seen. Thus, whenever there is fresh blood coming out in periods, it is seen as bright red blood. If there is mixing of the menstrual blood with cervical mucus, it takes on a jelly like consistency and starts looking pinkish in color because of the dilution. Ideally, the blood should be in a liquid state because the female body releases a lot of anti-coagulants along with it to keep it liquid and prevent from clotting. Occasionally, there is excessive bleeding; then there isn’t enough time to release adequate anti-coagulants and so there might be clots visible during the menstrual phase. All these variations are normal in females.
Summary: Whenever the menstrual blood comes from the higher end of the uterus and while it travels downward, it gets stored in the folds of the linings, it leads to formation of dark brown colored blood which is considered a normal variation. This is not a medical concern.