There are two main classifications of menstrual cramps: primary and secondary dysmenorrhea.
Primary Dysmenorrhea
This refers to common menstrual cramps that are caused by your monthly cycle itself, rather than an underlying disease. You may feel mild to severe pain in your lower abdomen, back, and thighs. It typically begins just before your period starts and usually lasts between 12 to 72 hours. Primary dysmenorrhea is more common in younger women and often becomes less severe from the mid-20s onward, and frequently improves after giving birth.
Secondary Dysmenorrhea
This type of dysmenorrhea is usually caused by an underlying disorder in a woman's reproductive organs. Some of these conditions include endometriosis, uterine fibroids, ovarian cysts, or pelvic infection. It can also be associated with the use of an intrauterine device (IUD) as a form of contraception. The pain associated with secondary dysmenorrhea often starts earlier in the menstrual cycle and tends to last longer than primary dysmenorrhea.