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Ovulation is an important part of the menstrual cycle in female vertebrates where the egg cells are released from the ovaries as part of the ovarian cycle. In female humans ovulation typically occurs near the midpoint in the menstrual cycle and after the follicular phase. Ovulation is stimulated by an increase in luteinizing hormone (LH).
Estrogen levels are highest right before the LH surge begins (Figure 1). The short-term drop in steroid hormones between the beginning of the LH surge and the event of ovulation may cause mid-cycle spotting or bleeding. [12] Under the influence of the preovulatory LH surge, the first meiotic division of the oocytes is completed.
Menstrual cycle The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that makes pregnancy possible. The ovarian cycle controls the production and release of eggs and the cyclic release of estrogen and progesterone. The uterine cycle governs the preparation and maintenance of the lining of the ...
In the 1920s, Kyusaku Ogino, a Japanese gynecologist, and Hermann Knaus, from Austria, working independently, each made the discovery that ovulation occurs about fourteen days before the next menstrual period. [13] Ogino used his discovery to develop a formula for use in aiding infertile women to time intercourse to achieve pregnancy.
The supply of follicles decreases slightly before birth, and to 500,000 by puberty for the average case (populations at puberty range from 25,000 to 1.5 million). [4] By virtue of the "inefficient" nature of folliculogenesis (discussed later), only 400–500 of these follicles will ever reach the preovulatory stage.
In a reference group representing all women, the 95% prediction interval of the LMP-to-ovulation is 8.2 to 20.5 days. [ 31 ] The average time to birth has been estimated to be 268 days (38 weeks and two days) from ovulation , with a standard deviation of 10 days or coefficient of variation of 3.7%.
Ovulation occurs ~35 hours after the beginning of the LH surge or ~10 hours following the LH surge. Several days after ovulation, the increasing amount of estrogen produced by the corpus luteum may cause one or two days of fertile cervical mucus, lower basal body temperatures, or both. This is known as a "secondary estrogen surge".
The average age of a girl's first period is 12 to 13 (12.5 years in the United States, [6] 12.72 in Canada, [7] 12.9 in the UK [8]) but, in postmenarchal girls, about 80% of the cycles are anovulatory in the first year after menarche, which declines to 50% in the third year, and to 10% by the sixth. [9]
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