Age of Onset of Menopause
Written by Dr.M.D.Mazumdar, MD
The average age of onset of menopause depends on genetics, race, family history and a number of other factors different in each woman.
This is because all women are born with a finite number of graafian follicles in their ovaries that are determined genetically.
Menopause occurs when this stock of follicles are exhausted at the end of the reproductive life.
Although the normal age of menopause can vary anywhere between 45 years to 55 years, the average age of menopause is 51 years.
Surgical menopause, of course , can occur at any time the uterus is surgically removed.
It is estimated that there are about 7 million primary oocytes or primary follicles in a 24 week old fetus. These decrease to about 2 million in number by the time the the fetus is born.
The number of oocytes continue to decrease gradually during childhood and at the time of the first menstrual period at about 11-12 years of age, the number of oocytes is about 300,000. Of these only about 500 oocytes are destined to develop into graafian follicles, a few every month, throughout the reproductive life. The rest are lost through degeneration.
Development of oocytes to mature follicles remain arrested from birth to puberty. At puberty, a group of oocytes begin to develop into mature ovum under the influence of hormones released from the pituitary gland in the brain. Of this group (called a 'cohort'), only one ovum is destined to mature fully and ovulate. The rest die off and degenerate inside the ovary.
By the time a woman has reached 25 years, the number of oocytes capable of developing into follicles become 60,000. And at the age of 40 years, the number of oocytes are less than 8000. This calculation is based on the assumption that the woman has menstruated regularly every month without break till the age of 40 years.
But most woman do not menstruate or ovulate regularly every month. There are other factors that come into play. For example, the number of pregnancies, the number of children a woman has given birth to, the years spent breastfeeding, years of taking birth control pills (the pills stop ovulation) - each of these factors prevent wastage of follicles and help build up the stock of follicles. This can delay menopause to some extent.
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During pregnancy , that is for 9 months, there is no ovulation. At least 5 - 6 follicles develop in each menstrual cycle during regular menstruation. So, each pregnancy means that there are at least 45 follicles (9x5=45) which remain in excess in the ovaries. The woman who has been pregnant more often will thus have a larger stock of follicles in her ovaries and will thus menstruate for a longer time.
Ovulation is also arrested in some women during the first 5-6 months of active breastfeeding, leading to a larger stock of follicles at around the age of 45 years. So, pregnancy and breastfeeding are likely to delay the onset of the age of menopause.
For example, let us assume that a daughter and her mother started menstruating when both of them were 12 years old. If the mother had 2 children and breastfed each for at least 1 year, it means that she had not ovulated for 4 years of her reproductive life ( 10 months of pregnancy plus approximately 1 year of breastfeeding).
If the daughter, on the other hand, has 3 children and has breastfed them all, it means that she has not ovulated for 6 years of her reproductive life. Naturally, her stock of follicles will be higher and she will get her menopause later than her mother.
Taking birth control pills which prevent ovulation will also delay menopause. Birth control pills contain hormones that can block the process of oocyte maturation and ovulation. This will again prevent decrease in the number of oocytes in the ovaries.
One important factor that should be noted is that lack of ovulation does not prevent the aging of oocytes within the ovary. Over time, oocytes will continue to be lost, even when there is no ovulation as in pregnancy, breastfeeding and taking of birth control pills. But this loss is substantially less than the loss that occurs when ovulation is actually taking place.
There are other factors that can also affect the age of onset of menopause. Women with the same ethnic background get their menopause at about the same time. For example, Japanese women are known to get their menopause later than women with African ancestors. This is probably because of a similar genetic component.
Stress, obesity, environmental factors, nutritional deficiencies can all affect the age of onset of menopause.
It has even been suggested that climatic conditions can affect the average age of onset of menopause. Women of colder climates generally get their menopause at a later age than women of tropical climates. But this is probably due to similar genetic composition of the women who stay at the same place(either colder or warmer climate ) together and may not be directly linked to the climate.
The average age of onset of menopause thus depends not only on the genetic makeup of the woman concerned but also on other factors which influence her life.
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