Prenatal testing is used to evaluate the health of your baby and check for birth defects. Most babies are born healthy, but a small number (1 percent) are born with major birth defects, and 3 to 4 percent are born with minor birth defects that may be caused by genetic, environmental or other factors.
Types of birth defects
The major categories of birth defects are chromosomal abnormalities, abnormalities involving a known mutation in a particular gene, structural birth defects that are not related to chromosomal abnormalities or a specific known gene, and birth defects due to certain maternal infections or environmental exposures.
Chromosomal abnormalities are described below. The best known of these is Down syndrome.
Many mutations in specific genes have been identified, with the number of birth defects for which a “DNA fingerprint” has been developed growing daily. Cystic fibrosis is one of the most common inherited single gene disorders. Tay-Sachs disease is one of the best-studied disorders, and is most common in individuals of Ashkenazi Jewish ancestry or French-Canadian/Cajun ancestry. Sickle cell disease is the most common single gene disorder in African-Americans, and alpha-thalassemia occurs most commonly in those of Asian ancestry. Maternal infections, including toxoplasmosis, syphilis and cytomegalovirus, can cause birth defects.
Toxoplasmosis is transmitted most commonly by eating raw or undercooked meat (steak tartare), but can also be transmitted to pregnant women by changing kitty litter. If you think you may have been exposed, please let your doctor know.
Syphilis screening is done with the first set of lab tests during the pregnancy.
Cytomegalovirus is typically transmitted by contact with saliva, urine, or other body fluids of an affected individual. Cytomegalovirus infection can cause abnormalities of the brain, intestines, or other organs of the baby.
Exposures to teratogens can also cause birth defects. The most common teratogen exposure during pregnancy is to alcohol. Certain drugs and medications are also teratogenic. We therefore recommend that women planning on pregnancy review all of the medications which they are taking, including over-the-counter medicines, vitamins, and herbal supplements, with their health care provider either before they become pregnant or very early on in the pregnancy.
What determines an individual’s appearance, hair color and eye color? Each cell of the developing embryo (and each cell of the adult) carries genetic information in chromosomes, which, in turn, have long sequences of DNA that serve as a sort of computer program of instructions for development. Normally, every cell of the body contains 46 chromosomes. For females, there will be 46 chromosomes, including two X chromosomes (chromosome count 46-XX), and for males, there will be 46 chromosomes, including an X chromosome and a Y chromosome (46-XY chromosome set).
The chromosomes are so important that individuals with missing or extra chromosomes usually have quite significant physical impairments. For example, Down syndrome is a condition in which an individual carries an extra copy of chromosome number 21. Individuals with Down syndrome have an unusual and characteristic facial appearance, moderate mental retardation, and short stature, and often have major internal abnormalities, such as heart defects or abnormalities of the formation of the trachea, esophagus or intestines.
An extra number 18 chromosome results in Edwards syndrome (trisomy 18), a much rarer disorder that results in miscarriage, stillbirth or death in early infancy more than 90 percent of the time. Individuals with Edwards syndrome have multiple physical abnormalities, including abnormalities of the brain, heart, and extremities, and are profoundly mentally retarded.
Patau syndrome (trisomy 13) results when an individual carries an extra copy of chromosome number 13. Individuals with Patau syndrome also have a very high rate of miscarriage, stillbirth and death in the newborn period. The few survivors are profoundly mentally retarded, and have major structural malformations, including brain, spine, heart, intestinal and limb abnormalities.
An extra or missing sex chromosome causes problems, but these are not as profound as Down syndrome, Edwards syndrome or Patau syndrome.
47XXX These are females with an extra X chromosome. There is a predisposition toward short stature, and women who have an extra X chromosome have an average IQ of 85 to 90, as opposed to the general population, which has an IQ average of about 100. However, most individuals with an extra X chromosome do not have identifiable physical abnormalities and function normally in society.
46XXY These are boys with an extra X chromosome. As for girls with an extra X chromosome, they have average IQs in the 85-90 range, and are typically free of major structural malformations. Boys with an extra X chromosome (Klinefelter syndrome) are typically tall and slim, and usually have problems with sperm production.
47XYY These are males with an extra Y chromosome. Older studies suggested that these individuals might be overrepresented in prison populations, and there was some concern that the presence of an extra Y chromosome predisposed toward violent behavior. More recent data suggests that individuals with an extra Y chromosome do not have any differences in the rate of physical or behavioral problems as opposed to individuals with the usual 46XY chromosome type.
45XO (Turner syndrome) This occurs when an embryo develops with only a single X chromosome. Individuals with Turner syndrome are typically short, have an unusual appearance to the neck, have an IQ averaging in the 85 range, and have an increased likelihood of certain cardiac abnormalities. Most women with Turner syndrome are also infertile.
There are many other types of chromosome abnormalities, including missing or extra fragments of chromosomes, or chromosomal rearrangements. If you have a chromosome problem in the family, then a genetic counselor at one of our centers can provide you with information about the specific implications for the development of the individual and for recurrence risk within families.
Chromosome problems are fairly frequent. Sex chromosomal abnormalities occur in about 2 per 1,000 births or about 1 in 500. Down syndrome occurs in about 1.6 per 1,000 births or about 1 in 600.
Some types of chromosome problems do not run in families and are unrelated to maternal age. Problems involving an extra chromosome, other than the sex chromosomes, including Down syndrome, Edwards syndrome, and Patau syndrome, are related to the mother’s age. The chance of Down syndrome, for example, is less than 1 in 1,000 below age 25, rising to about 1 in 300 at age 35, 1 in 86 at age 40 and 1 in 22 by age 45.