Conditions that cause large clots in the menstrual flow include fibroid tumors, thyroid problems and hormonal imbalances, says WebMD. What seems like a menstrual flow where clots are passed could also be a miscarriage.
If a woman believes she is pregnant and starts passing large blood clots, she should seek immediate medical attention, says WebMD.
Fibroid tumors of the uterus are benign, but they sometimes cause heavier than normal menstrual flow as well as clots, claims WebMD. Abnormalities in the levels of thyroid hormones also cause women to pass clots during their menstrual periods. An imbalance in estrogen and progesterone causes the uterine lining to become thicker than normal. When the lining is shed during the menstrual period, clots are also passed.