Intermenstrual bleeding, previously known as metrorrhagia, is uterine bleeding at irregular intervals, particularly between the expected menstrual periods. It is a cause of vaginal bleeding. In some women, menstrual spotting between periods occurs as a normal and harmless part of ovulation. Some women experience acute mid-cycle abdominal pain around the time of ovulation (sometimes referred to by the German term for this phenomenon, mittelschmerz). This may also occur at the same time as menstrual spotting. The term breakthrough bleeding or breakthrough spotting is usually used for women using hormonal contraceptives, such as IUDs or oral contraceptives, in which it refers to bleeding or spotting between any expected withdrawal bleedings, or bleeding or spotting at any time if none is expected. If spotting continues beyond the first 3-4 cycles of oral contraceptive use, a woman should have her prescription adjusted to a pill containing higher estrogen:progesterone ratio by either increasing the estrogen dose or decreasing the relative progestin dose.Besides the aforementioned physiologic forms, metrorrhagia may also represent abnormal uterine bleeding and be a sign of an underlying disorder, such as hormone imbalance, endometriosis, uterine fibroids, uterine cancer, or vaginal cancer. If the bleeding is repeated and heavy, it can cause significant iron-deficiency anemia. Cause
Intermittent spotting between periods can result from any of numerous reproductive system disorders:

Breakthrough bleeding
Breakthrough bleeding (BTB) is any of various forms of vaginal bleeding, usually referring to mid-cycle bleeding in users of combined oral contraceptives, as attributed to insufficient estrogens. It may also occur with other hormonal contraceptives.
PID or Pid may refer to:

Medicine
Pelvic inflammatory disease or pelvic inflammatory disorder, an infection of the upper part of the female reproductive system
Primary immune deficiency, disorders in which part of the bodys immune system is missing or does not function properly
Prolapsed intervertebral disc, commonly called a herniated disc

Science, technology and engineering
BBC Programme Identifier, a unique identifier for a BBC television or radio programme brand, a season or series, or an individual episode
OBD-II PIDs (on-board diagnostics parameter IDs), requests for data through an OBD connector in automotive repair
Packet Identifier, a field in a MPEG transport stream packet
Passive infrared detector, a passive infrared sensor
Persistent identifier, a long-lasting reference to a document, file, web page, or other object
Phosphotyrosine-interacting domain, also known as phosphotyrosine-binding domain, a protein domain which bind to phosphotyrosine
Photoionization detector, measures volatile organic compounds and other gases
Physical Interface Device, a class of a USB device
PID controller (proportional-integral-derivative controller), a control concept used in automation
Piping and instrumentation diagram (P&ID), a diagram in the process industry which shows the piping of the process flow etc.