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Down's syndrome occurs when either the egg or the sperm cell contain on extra chromosome 21. To my understanding, women are born with all the egg cells in place already, so there's no further cell divisions, and thus no further chance of having one cell with an extra chromosome.

Now, the chance of having a baby with Down's syndrome increase with age:

At maternal age 20 to 24, the probability is one in 1562; at age 35 to 39 the probability is one in 214, and above age 45 the probability is one in 19

Why is this so, when the egg cells are complete at birth?

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It is true that woman are born with all the eggs they will ever release , meiosis is not completed in the egg. There will be a meiotic cell division. Now this might sound confusing to you how there can be cell division without increase of egg number. For this - I will recommend that you read any basic bio textbook covering both the meiosis and gamete formation in females. –  biogirl Dec 7 '13 at 18:35
    
I think by this discussion it is clear that there is a chance of having one cell with extra chromosome. –  biogirl Dec 7 '13 at 18:40
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1 Answer

up vote 5 down vote accepted

The frequency rises with maternal age due to a peculiarity of meoisis in female mammals. Meiosis is originated in the fetal ovary, arresting at metaphase I with the homologous chromosomes aligned for segregation. Cells remain in this state until the time of ovulation, often decades later in humans. The longer cells remain in the arrested state, the greater the chance that there will be a nondisjunction event when meiosis resumes.

Source : http://www.madsci.org/posts/archives/2005-07/1121714807.Ge.r.html

I think you have the misconception that egg cells have completed meiosis by the time female is born. This is not true ( as indicated above in the answer). At birth, they are in metaphase I , after puberty, each month some of them divide further but only one survives and get arrested at metaphase II . Only after fertilization is the meiosis complete.

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If you are not familiar with some of the terms or have any other confusion , ask ! –  biogirl Dec 7 '13 at 18:33
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