The prevalence of giving birth to a baby with Down syndrome is significantly higher in older women (over the age of 35) because as a woman ages, the probability that one of her eggs will contain an excess copy of the 21st chromosome increases greatly. Nearly ½ of babies with Down syndrome are born to women over the age of 35 every year in the United States. 

The Age of the Mother

When one or both of the parents has Down syndrome or another variation of chromosomal abnormality, their child is more likely to inherit a chromosomal abnormality, of which Down syndrome is the most common. 

If one or both parents have Down syndrome or a chromosomal abnormality

Though the occurrence of Down syndrome is random and not directly correlated with any health, personal or socioeconomic factors, there are two certain characteristics of a parent that increase the likelihood of a child being born with Down syndrome. These two characteristics are: 
- The age of the mother 
- If one or both parents have Down syndrome or a chromosomal abnormality 



University of Virginia, Charlottesville

Down syndrome is caused by the presence of an extra copy of the 23rd chromosome, resulting in an excessive number of chromosomes: 47, as opposed to 46. However, variations of this chromosomal abnormality exist. And though it is the most common chromosomal abnormality, births with Down syndrome are random and not linked to any health or socioeconomic patterns. We can best explore the variations of the occurrence of Down syndrome through:  
- Types of Down syndrome
- Trisomy 21
- Frequency 
- Inheritance 
- Factors that increase risk 

What causes Down syndrome?

The majority of Down syndrome cases are not inherited, but there are two ways in which a fetus can acquire the genetic condition that causes Down syndrome. These two ways occur through:

1. A random event during fertilization or early fetal development 
2. An unaffected parent being a carrier of a particular chromosomal rearrangement 


Inheritance

There are three types of Down syndrome, distinguished by the form in which chromosome 21 is copied. The three types are: 
- Trisomy 21 (also known as nondisjunction):
- Translocation
- Mosaicism 


Types of Down syndrome

Factors that Increase Risk 

According to the CDC, approximately 1 in every 772 babies are born with Down syndrome in the U.S. every year, making it the most common chromosomal condition.

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