This could be due to older pieces of uterine lining and blood which could be seeking an outlet with your flow but this is normal. According to Doctors, there is no real reason why it happens but it is possible that the blood may have been there for some time and comes out slowly. The brown or blackish color is because of oxidation that happens during this time. Seeing dark brown period blood during the onset or the end of a period is normal.
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This is an indication of low progesterone levels ad high estrogen levels. Sometimes clotting could be normal but if you find large clots in your period flow, that could mean a hormonal imbalance. You could also try reducing your intake of dairy, sugar and soy to try and see if there is a difference.
Uterine Fibroids could also be possible in such cases which are usually benign but a bit painful. If you find your periods full of clots and suspect that fibroids could be the case, consult your doctor for an ultrasound scan.
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