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

While the occurrence of Down syndrome is generally random and not directly correlated with most health, personal, or socioeconomic factors, certain parental characteristics can increase the likelihood of a child being born with the condition. The two primary risk factors are advanced maternal age and having one or both parents who carry a chromosomal abnormality or have Down syndrome.

University of Virginia, Charlottesville

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Down syndrome is caused by a chromosomal abnormality involving an extra copy of chromosome 21, resulting in an excessive number of chromosomes (47 instead of the typical 46). Although variations of this chromosomal abnormality exist and it is the most common chromosomal condition, occurrences of Down syndrome are generally random and not linked to socioeconomic patterns. Its etiology can be further understood by examining the specific genetic types, inheritance patterns, and associated risk factors.

Causes of 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

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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