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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]
This decreases to around 2 million by the time a girl is born, and 300,000 by the time she has her first period. On average, one egg matures and is released during ovulation each month after menarche. [17] Beginning at puberty, these mature to primary follicles independently of the menstrual cycle. [18]
The first day of menstrual bleeding is the date used for the last menstrual period (LMP). The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, and 21 to 35 days in adults. [2] [3] The average length is 28 days; one study estimated it at 29.3 days. [10]
The spindle apparatus of the second meiotic division appears at the time of ovulation. If no fertilization occurs, the oocyte will degenerate between 12 and 24 hours after ovulation. [18] Approximately 1–2% of ovulations release more than one oocyte. This tendency increases with maternal age.
Chance of fertilization by menstrual cycle day relative to ovulation [14] Pregnancy rates for sexual intercourse are highest during the menstrual cycle time from some 5 days before until 1 to 2 days after ovulation. [15] For optimal pregnancy chance, there are recommendations of sexual intercourse every 1 or 2 days, [16] or every 2 or 3 days. [17]
Ovulation occurs 24–48 hours after the luteinizing hormone peak, which occurs around the fourth day of estrus; therefore, this is the best time to begin breeding. Proestrus bleeding in dogs is common and is believed to be caused by diapedesis of red blood cells from the blood vessels due to the increase of the estradiol-17β hormone .
In a reference group representing all women, the 95% prediction interval of the LMP-to-ovulation is 8.2 to 20.5 days. [30] 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%. [32]
The Standard Days Method is based on the fact that there is a fertile window during a woman's menstrual cycle which begins several days before ovulation and ends a few hours after ovulation. During this time a woman can become pregnant. The Standard Days Method identifies days 8-19 of cycle for women with cycles between 26 and 32 days long, as ...