Dysfunctional Uterine Bleeding (DUB): This is the most common cause of abnormal uterine bleeding in women of reproductive age, often related to hormonal imbalance, particularly anovulatory cycles leading to unopposed estrogen effect.

Uterine Fibroids: These can cause irregular and heavy menstrual bleeding due to their location, size, and number. They are common in reproductive-aged women and can lead to prolonged or irregular bleeding.
Endometrial Hyperplasia: This condition, often associated with unopposed estrogen, can lead to irregular and prolonged bleeding. It's more common in perimenopausal women but can occur in any age group with risk factors for estrogen excess.
Intrauterine Device (IUD) Complications: If the patient has an IUD, especially a non-hormonal one, it could be a cause of irregular bleeding, especially in the initial months after insertion.
Thyroid Dysfunction: Both hyperthyroidism and hypothyroidism can lead to menstrual irregularities, including prolonged or intermenstrual bleeding, due to the effect of thyroid hormones on the menstrual cycle.

Endometrial Cancer: Although less common, endometrial cancer is a critical diagnosis not to miss, especially in older women or those with risk factors such as obesity, diabetes, or a family history of endometrial or colorectal cancer. Irregular vaginal bleeding is a key symptom.
Cervical Cancer: Abnormal vaginal bleeding, including bleeding between periods, can be a symptom of cervical cancer. Given its potential severity, it's crucial to consider and rule out with appropriate screening.
Pregnancy Complications: Ectopic pregnancy or a spontaneous abortion can present with irregular bleeding and must be considered, especially if there's a possibility of pregnancy.

Bleeding Disorders: Conditions like von Willebrand disease can lead to heavy or prolonged menstrual bleeding. While not common, they should be considered, especially in women with a significant history of bleeding problems.
Uterine Arteriovenous Malformations (AVMs): These are rare vascular lesions that can cause abnormal uterine bleeding due to the direct connection between arteries and veins, bypassing the capillary bed.
Ovarian Tumors: Some ovarian tumors can produce hormones leading to endometrial hyperplasia and subsequent irregular bleeding. They are less common but should be considered in the differential diagnosis, especially with other symptoms like pelvic mass or pain.