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What is Mittelschmerz and explain in detail?
Mittelschmerz (German: "middle pain") is a colloquial term for "ovulation pain" or "midcycle pain". About 20% of women experience mittelschmerz, some every cycle, some intermittently.

Signs and symptoms
Mittelschmerz is characterized by lower abdominal and pelvic pain that occurs roughly midway through a womans menstrual cycle. The pain can appear suddenly and usually subsides within hours, although it may sometimes last two or three days. In some cases it can last up to the following cycle. In some women, the mittelschmerz is localized enough so that they can tell which of their two ovaries provided the egg in a given month.  Because ovulation occurs on a random ovary each cycle, the pain may switch sides or stay on the same side from one cycle to another.

Other ovulation symptoms
Women may notice other physical symptoms associated with their mittelschmerz, during or near ovulation.  The most common sign is the appearance of fertile cervical mucus in the days leading up to ovulation.  Cervical mucus is one of the primary signs used by various fertility awareness methods.  Other symptoms are sometimes called secondary fertility signs to distinguish from the three primary signs.
Mid-cycle or ovulatory bleeding is thought to result from the sudden drop in estrogen that occurs just before ovulation.  This drop in hormones can trigger withdrawal bleeding in the same way that switching from active to placebo birth control pills does.  The rise in hormones that occurs after ovulation prevents such mid-cycle spotting from becoming as heavy or long lasting as a typical menstruation.  Spotting is more common in longer cycles.
A womans vulva may swell just prior to ovulation, especially the side on which ovulation will occur.
One of the groin lymph nodes (on the side on which ovulation will occur) will swell to about the size of a pea, and may become tender.

Causes
Mittelschmerz is believed to have a variety of causes:

Follicular swelling: The swelling of follicles in the ovaries prior to ovulation