The passing of blood clots during a period is normal, because the thickened uterus lining sheds. Other reasons for clotting during menstruation can include certain lifestyle changes, such as weight loss or medication. A doctor's visit might be necessary if the frequency of blood clots increase, because serious health problems could be developing.

A heavy flow is among the reasons for passing blood clots during a period. Anticoagulants that the body produces to allow blood flow and prevent clots are not as effective during heavy menstrual cycles. As a result, clotting is likely to occur and might cause the menstrual bleeding to seem thicker.

Miscarriage, the termination of a pregnancy, can also be one of the reasons for passing blood clots during a period. In the case of a miscarriage where disorders such as lupus are evident, clotting prevents the placenta from receiving blood. Over-the-counter medications such as baby aspirin can be used to thin the blood and lower the risk of clots in these circumstances.

Fibroid tumors that grow in the uterus also contribute to the passing of blood clots during a period. These non-cancerous tumors are likely to cause heavy and abnormal menstrual bleeding. As a result, fibroids increase the amount of clots that form during menstruation.

A hormonal imbalance of progesterone and estrogen is another reason for passing clots during a period. The progesterone hormone is produced by the ovaries to help the uterus prepare for fertilization when an egg is released. The estrogen hormone aids in the reproduction process, the development of female sexual characteristics and the regulation of a menstrual cycle. The balance between progesterone and estrogen hormones can be disturbed because of a variety of factors including menopause, medication, significant weight loss or gain, benign polyps and endometriosis. When an imbalance occurs between these hormones, the lining of the uterus becomes thick and causes heavier bleeding, which heightens the development of clots during a period.

Pelvic inflammatory disease (PID) might be one of the more serious reasons for passing blood clots during menstruation. This disease is an infection of the reproductive organs, including the fallopian tubes, uterus and cervix. PID causes abnormal menstrual bleeding and is associated with sexually transmitted diseases, including gonorrhea and chlamydia.

Birth control methods such as oral contraception, the use of an intrauterine device (IUD) and the use of an injected contraceptive are additional reasons that might cause the passing of blood clots during a period. The side effects of these birth control methods include heavier, irregular and prolonged menstrual bleeding. This increases the risk of clotting.