Other birth defects include heart defects, cleft lip with or without cleft palate, and others, many of which cannot be diagnosed until after birth.
Prenatal diagnostic procedures include amniocentesis and chorionic villus sampling (CVS). These procedures are described in detail in this section. Amniocentesis and CVS are procedures that can detect fetal chromosome abnormalities and certain other specific birth defects.
Below you will find further information about conditions that prenatal diagnostic procedures can often detect before birth.
Chromosome
Abnormalities
Chromosomes are the packages of genetic information that we inherit from our parents and pass on to our children (please visit our Genetics 101 section for more information). Chromosomes are found in every cell of the body and contain the genes, which are responsible for growth and development. Usually, healthy people have 46 chromosomes in every cell, 23 pairs.
A chromosome abnormality happens when a baby is born with some change in the number or structure of the chromosomes. This leads to the individual having extra or missing chromosomal material, known as a chromosome abnormality. Many chromosome abnormalities cause birth defects and mental retardation.
Down syndrome is the most common chromosomal disorder. It happens in about 1 out of every 800 births. Down syndrome is also known as trisomy 21. It occurs when there is an extra, or third copy of chromosome #21. Features of Down syndrome include moderate mental retardation and a typical facial appearance. About 40% of individuals with Down syndrome also have heart defects. Other less common chromosome abnormalities may be more or less severe than Down syndrome.
Other chromosome abnormalities
Trisomy 18 and trisomy 13 are other chromosome abnormalities. They occur in about 1/4000 births. They are caused by an extra, or third copy of chromosome number 18 or 13 in all of the cells in the body. The extra copy of chromosome 18 or 13 is present from the time of conception. Babies with trisomy 18 and 13 have many birth defects including heart, brain, and kidney abnormalities. Babies with trisomy 18 and 13 seldom live longer than a few days or weeks. Many die before they are born.
Chance
of Having a Baby with a Chromosome Abnormality
Most chromosome abnormalities (including most cases of Down syndrome) are not hereditary, that is, they do not run in families. Older women have a greater risk than younger women for having a baby with a chromosome abnormality. Although the risk gradually increases with age, a woman of any age can have a baby with a chromosome abnormality.
The following chart shows the approximate chances of having a baby with a chromosome abnormality:
Mother's Age at
Due Date |
Approximate chance of a having a baby with a chromosome abnormality |
20 |
1/525 |
25 |
1/475 |
30 |
1/400 |
35 |
1/200 |
36 |
1/165 |
37 |
1/125 |
38 |
1/100 |
39 |
1/80 |
40 |
1/65 |
41 |
1/50 |
42 |
1/40 |
43 |
1/30 |
44 |
1/25 |
>45 |
1/20 |
Prenatal diagnostic procedures to detect chromosome abnormalities
Amniocentesis and CVS (chorionic villus sampling) are diagnostic procedures that can detect a chromosome abnormality before birth. Both procedures have a small risk of miscarriage or loss of the baby (less than 1 in 100). These procedures are described in detail below.
Some women decide that even though there is a small risk for miscarriage, they would like to have a prenatal diagnosis procedure to find out whether the baby has a chromosome abnormality.
Other women only choose a diagnostic procedure after having a screen positive first or second trimester screening test (FTS or Expanded AFP screening). To learn about the differences between screening tests and diagnostic procedures, please visit Prenatal Testing for Birth Defects: A Guide to Helping You Decide .
Neural Tube Defects (NTDs)
The “neural tube” is another name for the structure that forms the brain and spine very early in pregnancy (about 3-4 weeks after conception). Neural tube defects (NTDs) are birth defects that happen when this part of the baby’s body does not form correctly. About 1 in 1000 babies are born with a neural tube defect. Neural tube defects are not related to the age of the mother, and usually do not run in families. Folic acid, a B vitamin, has been shown to be important in the prevention of some neural tube defects.
Types of neural tube defects:
Spina bifida happens if the spine does not close properly and some of the nerves and blood vessels are not covered by skin. This can cause damage to the nerves resulting in weakness or loss of feeling in the legs, or problems with being able to go to the bathroom normally.
Anencephaly happens when the brain and the top of the skull do not develop normally. This is the most severe form of neural tube defect. Babies with anencephaly cannot survive after birth. Many die before birth or are born dead (stillborn).
Detection of neural tube defects:
Amniocentesis can detect most neural tube defects. Neural tube defects can also be detected prenatally by blood screening tests such as Expanded AFP blood screening and by ultrasound examination.
Folic acid is an important B vitamin that has been shown to be helpful in preventing some neural tube defects.
Other
conditions detectable by prenatal diagnosis
Chromosome abnormalities and neural tube defects are the most common conditions that we look for with routine prenatal diagnosis procedures. However, some other conditions are available using similar techniques.
For some families, if there is a specific genetic disease that is known to run in the family, there may be a chance that prenatal testing is available. This is dependent upon the specific condition, the risk to the fetus, and the availability of genetic testing for that condition.
Testing for some for common autosomal recessive conditions is available. For example, if both parents are known to be carriers for cystic fibrosis or sickle cell disease, testing the pregnancy may be possible.
If you have concerns about these issues or feel that you qualify for prenatal testing for one of these conditions, we suggest you contact a Genetic Counselor for more information.
Diagnostic Procedures
CVS and Amniocentesis
CVS (Chorionic Villus Sampling)
CVS is a prenatal diagnostic procedure that is usually done between 10 and 13 weeks of pregnancy (during the first trimester). Ultrasound is used during the procedure. It involves removing and analyzing a sample of the developing placenta (the chorionic villi) to check for chromosome abnormalities in the developing baby. There are two methods for doing CVS. The doctor doing the procedure will decide which method is best for each patient.
Transcervical CVS is done by inserting a thin flexible tube (a catheter) through the vagina and cervix into the uterus and removing a small sample of the placental tissue (chorionic villi).
Transabdominal CVS is done by inserting a thin needle through the woman's abdominal wall into the uterus and removing a small sample of the placental tissue (chorionic villi).
Risks: The risk for miscarriage after CVS is less than 1 in 100. In about 3-5% of CVS tests, the result is unclear, and an amniocentesis is recommended (see below). In up to 15% of CVS tests, a patient may also be asked to provide a blood test to help confirm the results.
Results: The results indicate whether or not the baby has a chromosome abnormality such as Down syndrome or trisomy 18, for example. Results are usually available within 14 days.
CVS does not detect neural tube defects (NTDs) since the amniotic fluid containing AFP is not sampled. However, AFP can be measured in the woman's blood between 15 and 20 weeks of pregnancy by a simple blood test. A follow-up ultrasound or amniocentesis is offered if the level of AFP is elevated.
Here is a brief, easy-to-read fact sheet about CVS: Chorionic Villus Sampling.
Amniocentesis/Early Amniocentesis
Amniocentesis is a prenatal diagnostic procedure that is usually done between 15 and 20 weeks of pregnancy (during the second trimester). Ultrasound is used during the procedure. When amniocentesis is done between 14 and 15 weeks it is known as "early amniocentesis" (see below). It involves removing and analyzing a sample of the fluid that surrounds the baby to check for chromosome abnormalities in the baby.
A thin needle is inserted through the woman's abdominal wall and into the amniotic sac. A small amount of the amniotic fluid surrounding the fetus is removed. Cells from the the baby are floating in the amniotic fluid. These cells are grown in the laboratory, and the fetal chromosomes are studied. The amount of AFP in the amniotic fluid is also measured to look for neural tube defects and abdominal wall defects.
Risks: The risk for miscarriage after amniocentesis is less than 1 in 300.
Results: The results indicate whether or not the baby has a chromosome abnormality such as Down syndrome or trisomy 18, for example. Results are usually available within 14 days.
Early Amniocentesis
Early amniocentesis is the same procedure as amniocentesis, but it is done between 14 and 15 weeks of pregnancy. The risk of miscarriage is higher with early amniocentesis; less than 1 in 100.
Here is a brief, easy-to-read fact sheet about amniocentesis: Amniocentesis.
Results of Amniocentesis/Early Amniocentesis, CVS
Results from amniocentesis or CVS are ready in about two weeks. A genetic counselor will call you with your results as soon as they are available. Most amniocentesis results are normal, however, once in a while a chromosomal abnormality or other serious problem is found. Chromosome abnormalities are not curable. If amniocentesis or CVS results show that your baby has a birth defect or chromosome abnormality, you will be given more information about the specific problem and how it may affect the baby. All available treatments and options for continuing or ending the pregnancy will be discussed at that time. This may include a discussion with your genetic counselor, your doctor, nurse practitioner or nurse midwife, and possibly other specialists such as a geneticist or perinatologist.
Prenatal Ultrasound
Ultrasound is the use of sound waves directed at the developing baby and then made into an image on a video screen.
Ultrasound can provide important information about your pregnancy. Such as:
- determine the expected delivery date of the baby
- find out whether there is more than one baby (twins or triplets)
- check the location and development of the placenta and the baby
- measure the amount of amniotic fluid
- detect some birth defects
- learn if there has been a miscarriage
Ultrasound is also used during diagnostic procedures like CVS and amniocentesis.
Your doctor, nurse practitioner or nurse midwife will probably perform a brief ultrasound in the office early in your pregnancy to help confirm your due date and count the number of fetuses.
A second-trimester ultrasound, done in the ultrasound department at about18-20 weeks of pregnancy, is routinely offered to all pregnant women. This ultrasound may also be called a “fetal anatomy survey”or a “level 1 scan”. The exam will include a careful head-to-toe evaluation of the fetus to make sure the anatomy looks normal. The sonographer also evaluates the position of the fetus, the location of the placenta, and the amount of amniotic fluid. Often the baby’s gender (boy or girl) can be determined during this ultrasound, however, this is not the main purpose of the ultrasound.
Risks: Ultrasound has been routinely used in pregnancy since the 1960s. Based on a large number of medical studies, there are no known health risks to the fetus or mother associated with prenatal ultrasound.
Results: Most prenatal ultrasound results are normal. Your doctor, nurse practitioner or nurse midwife will discuss your routine ultrasound results with you.
Some women may be offered follow-up tests after their ultrasound.
A more detailed ultrasound, sometimes called a level 2 ultrasound or high resolution ultrasound, may be offered when the initial scan shows questionable findings or abnormalities, when the fetal anatomy is not well-visualized on the routine scan, or in the case of a positive screening test.
Some common ultrasound findings include:
- Echogenic intracardiac focus
- Single umbilical artery
- (Dilated renal pelvis)
- (Increased nuchal translucency [NT])
Although a normal scan is reassuring, it does not guarantee that the baby will be completely normal. Prenatal ultrasound cannot diagnose all malformations and problems of an unborn baby. Some abnormalities are very difficult to find or to be absolutely certain about, while other abnormalities may not be detectable until later in pregnancy, past the time of routine ultrasound.
Deciding whether to have a prenatal diagnostic procedure
The decision to have a prenatal diagnostic procedure is a very personal one. You may want to discuss the risks, benefits and limitations of such testing with your doctor, nurse practitioner, nurse midwife, or a genetic counselor. You may also want to visit Prenatal Testing for Birth Defects: A Guide to Helping You Decide for more information.
Keep in mind:
- Prenatal diagnostic procedures can provide you with some information about the baby's health before birth, such as whether the baby has a chromosome abnomality.
- Chromosome abnormalities are not curable.
- Prenatal diagnostic procedures have a small (less than 1 in 100) risk for miscarriage or loss of the pregnancy.
- There are no prenatal diagnostic procedures that can detect all birth defects before birth.
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