A woman's egg production actually starts in the early embryonic stage, and it is known that even in early pregnancy there are several small eggs. However, a natural reduction in the number of eggs is seen in the embryonic stage and at delivery a newborn baby girl’s ovaries normally contain up to one million eggs. When the girl reaches puberty, her ovaries contain a total of about 400,000 eggs, of which only 300-500 will ripen fully and can be fertilized. The other eggs gradually die off without ripening or being released. When a woman’s egg supply is used up, menopause ensues and her periods stop. This typically happens when the woman is aged about 45-50, but this can vary from woman to woman.
The first menstrual period is the start of the fertile age, and in the Western world this generally occurs when the girl is about 12-13.
A menstrual cycle lasts 28 days on average and begins when the pituitary gland, a gland in the brain secretes the hormone FSH (follicle-stimulating hormone). FSH stimulates the ovaries to begin ripening 8-10 follicles. A follicle is a fluid-filled sac that contains an egg. On around Day 7 of the cycle, there is usually one dominant follicle, i.e. a follicle which is a bit bigger than the others, and this is most likely to be the follicle that ruptures and releases an egg later on in the cycle. This follicle will suppress growth of the other follicles, and they die off. The dominant follicle will continue to grow, and the egg inside it will ripen while the follicular cells (granulosa cells)/ovaries produce the hormone oestrogen.
Oestrogen prevents the pituitary gland continuing its production of follicle-stimulating hormone so that no more eggs ripen during the current cycle. When oestrogen reaches a certain level, the ovulation hormone, luteinising hormone (LH), is released from the pituitary gland. Oestrogen also causes the endometrium to thicken so that the lining is ready to receive the egg that will emerge from the follicle around 14 days before the first day of the woman’s period.
Within around 24-36 hours of the release of LH from the pituitary gland, the egg will undergo the final ripening process (release of polar bodies), the follicle ruptures and the egg is released and sent on its way with follicular fluid. The egg is transported via the fallopian tubes by small fine fimbriae. For the woman to become pregnant, the egg, which is barely visible to the naked eye, has to meet sperm cells in the fallopian tube within a few hours of ovulation. A normal sperm cell has to penetrate the egg in the fallopian tube, and the fertilised egg must pass through the fallopian tube and down into the uterus, where must implant into the endometrium.
The follicle that contained the egg now becomes the "corpus luteum” or "yellow body”. The corpus luteum now begins to produce the hormone progesterone, which continues the development of the endometrium to prepare it to receive the fertilised egg, which then arrives in the uterus and implants about a week after ovulation.
If the egg is not fertilised, the corpus luteum dries out and the production of progesterone stops. This results in the endometrium breaking down and the start of another period. This process is repeated month after month until the menopause.
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