While hormonal imbalances are the most common cause of watery or thin blood during menstruation, several other factors could play a part. Other causes may include malnutrition, thyroid imbalance, a woman's age, a very thin uterine lining or early pregnancy. Taking birth control pills can also cause hormonal changes that impact the consistency of menstrual blood, according to The Access Project.
A watery period can be caused by implantation bleeding approximately six days after conception. This type of period will appear pink or light red and occurs in 30% of pregnancies, according to What to Expect.
Emotional or physical stress, such as that caused by heavy exercise, can also cause light, watery periods according to The Access Project. As more blood travels to the brain, heart and lungs, less is available for a woman's period. If light, watery periods continue for more than two or three cycles, The Access Project recommends seeing a gynecologist.
A much less common cause of watery periods can be intrauterine adhesions, also known as Asherman syndrome, according to the American Society for Reproductive Medicine. Asherman syndrome is usually caused by injury and results in scarring in the uterine cavity.