Numerical abnormalities are a type of chromosome abnormality. These types of birth defects occur when there is a different number of chromosomes in the cells of the body from what is usually found. So, instead of the usual 46 chromosomes in each cell of the body, there may be 45 or 47 chromosomes. Having too many or too few chromosomes may cause health problems or birth defects.
The term "trisomy" is used to describe the presence of three chromosomes, rather than the usual pair of chromosomes. For example, if your baby is born with three #21 chromosomes, rather than the usual pair, your baby would be said to have "trisomy 21." Trisomy 21 is also known as Down syndrome. Other examples of trisomy include trisomy 18 and trisomy 13. Again, trisomy 18 or trisomy 13 simply means there are three copies of the #18 chromosome (or of the #13 chromosome) present in each cell of the body, rather than the usual pair.
The term "monosomy" is used to describe the absence of one member of a pair of chromosomes. Therefore, there is a total of 45 chromosomes in each cell of the body, rather than 46. For example, if your baby is born with only one X sex chromosome, rather than the usual pair (either two X's or one X and one Y sex chromosome), your baby would be said to have "monosomy X." Monosomy X is also known as Turner syndrome.