Women are born with all of the eggs, which they will use in their lifetime. The eggs age with the individual and as women get older, their risk increases for having a chromosomally abnormal pregnancy. Once a woman is 35 year old at the time of delivery, she is considered to be of advanced maternal age.
Chromosomes are the inherited structures in the cells of the body. Normally, there are 46 chromosomes in each cell, arranged into 23 pairs. Chromosomal abnormalities involving an entire missing or extra chromosome are not inherited and are not caused by an exposure during pregnancy. Instead, they are caused by random mistakes in cell division at the time of conception and can occur in anyone's pregnancy. The chance for such an event to occur does increase with a woman's age.
The most common chromosomal abnormality in liveborns is Down syndrome (trisomy 21). Down syndrome is caused by an extra chromosome #21; thus, an affected individual has three rather than two copies of chromosome #21. It is this extra genetic material that causes the features of Down syndrome, including intellectual disability, a characteristic facial appearance, and other health problems.
Two other chromosomal abnormalities which are more likely to occur in women of advanced maternal age are trisomy 18 and trisomy 13, resulting from three copies of chromosomes #18 or #13, respectively. Babies with these chromosomal abnormalities have severe intellectual disability and serious birth defects. Most infants with these birth defects do not survive their first year of life. Although more severe than Down syndrome, they are also far less common. Other chromosomal abnormalities seen in liveborns occur with the sex chromosomes, the X and the Y. An extra or missing X or Y chromosome may cause mild physical differences, learning disabilities, behavioral and sometimes fertility issues.