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Prenatal Diagnosis

Most babies are born healthy, however approximately 3-4 in 100, or 3-4%, of babies are born with some type of birth defect ranging from mild to severe.  Birth defects can occur for a variety of reasons.  They can be caused by genetic (inherited), environmental, or unknown factors.  Couples may be at a slightly increased risk to have a baby with a birth defect due to risk factors such as maternal age, family history, or exposure to infectious or environmental triggers.

Types of Congenital Disorders

Chromosomal abnormalities
One type of birth defect, called a chromosome abnormality, can result in physical birth defects, mental retardation, or both.  Chromosomes are structures found in the center (the nucleus) of cells and contain genetic material, or genes.  This material is the building block of the human body.  Typically, humans have 46 chromosomes in every cell arranged into 23 pairs.  Half come from the mother’s egg and half come from the father’s sperm.  Males have one X and one Y chromosome, and females have two X chromosomes.

Chromosomal abnormalities occur when there is variation in either the number or structure of chromosomes.  As mentioned above, typically there are 46 chromosomes – 23 from the mother’s egg and 23 from the father’s sperm. Occasionally, the egg can contain an extra or missing chromosome, which gets passed on to the developing fetus after fertilization.  A common example of this type of chromosome abnormality is Down syndrome (also known as trisomy 21), which occurs when there are three, instead of two, copies of chromosome #21 in every cell in the body. 

It is possible to have an extra chromosome in pairs other than pair #21.  For example, trisomy 18 (Edward syndrome) occurs when there are three copies of chromosome #18. While a woman of any age has a risk for a chromosome abnormality, the risk increases as she gets older. 

Common single gene defects

Cystic Fibrosis (CF)
CF is an inherited disease that affects about 30,000 American children and adults.  CF does not affect intelligence. The symptoms of CF are variable and can include lung congestion, pneumonia, poor growth, and diarrhea. Many people with CF have severe medical problems and some die at an early age from complications. Other people with CF have very few symptoms and may not be aware that they have the condition.  There is no cure for CF at this time. However, there have been significant advances in treatment and today many individuals with CF live into their 20’s and 30’s. 

The chance to have a baby with CF varies depending upon your ethnic background.  It is important to note that it possible to have a child with CF even if there is no family history. The inheritance is autosomal recessive, which means that both parents must be carriers to have a 1 in 4, or 25%, chance in each pregnancy to have a baby with CF.  Carriers do not typically show clinical signs of CF and therefore are undetected without carrier screening.  The carrier frequency for individuals of Caucasian descent is approximately 1 in 25.  Although individuals of Caucasian descent have the highest carrier frequency, individuals from other ethnic groups can also be carriers for CF.  Table 1 shows the likelihood of having a child with CF based on ethnic background.

For more information on cystic fibrosis, click here.

Ashkenazi Jewish Panel
There are a group of disorders that are known to run at a higher frequency in the Ashkenazi (Eastern European) Jewish population.  Although some of the disorders in this group are considered rare, the overall likelihood that an individual of Ashkenazi Jewish descent is a carrier for one of these disorders is approximately 1 in 5. All of the disorders included in this group can cause variable mental retardation, physical impairment, and/or shortened lifespan.

Currently, the American College of Obstetricians and Gynecologists recommends that screening be offered to couples of whole or partial Ashkenazi Jewish descent for the following disorders.

• Tay-Sachs
• Cystic fibrosis
• Canavan disease
• Familial dysautonomia

There are a number of other disorders which are included in an expanded panel for which carrier testing is available.  In January of 2008, the American College of Medical Genetics came out with the following recommendations for carrier screening for individuals of Ashkenazi Jewish descent.

1. Carrier screening should include:
• Tay-Sachs
• Cystic fibrosis
• Canavan disease
• Familial dysautonomia
• Fanconi anemia (Group C)
• Niemann-Pick (Type A)
• Bloom syndrome
• Mucolipidosis IV
• Gaucher disease

2. Screening should ideally take place before pregnancy, allowing individuals time to make reproductive choices.
3. If one member of a couple is of Ashkenazi Jewish descent, testing should still be offered.  The member of the couple that is of Jewish descent should be tested first.  If there is a positive test result, than the partner should be tested for the specific disorder regardless of ethnic background.

For more information on carrier testing and the specific disorders, click here.

Hemoglobinopathies
The hemoglobinopathies are a diverse group of disorders caused by the presence of abnormal hemoglobin or decreased production of hemoglobin in the blood.  Hemoglobin is the substance that carries oxygen in the blood and gives blood its red color. The severity of these disorders ranges from mild to severe, depending on the specific type of hemoglobinopathy.  Hemoglobinopathies tend to run at a higher frequency in individuals of African, Mediterranean, Middle Eastern, or Southeast Asian descent.  The inheritance is autosomal recessive, which means that both parents must be carriers to have a 1 in 4, or 25%, chance in each pregnancy to have a baby with a hemoglobinopathy.  Carrier screening for hemoglobinopathies can be performed by simple blood tests at your doctor’s office or at SFPA.  Table 2 shows the carrier frequency based on ethnic background for some of the common hemoglobinopathies.

For more information on hemoglobinopathies, click here.

Fragile X syndrome
Fragile X syndrome is the most common cause of inherited mental retardation, occurs among people of all ethnic backgrounds, and affects males more severely than females.  Approximately 1/4000 males are affected and 1/8000 females show significant signs of the disorder.  The clinical findings of fragile X include variable degrees of mental retardation or learning disabilities, and characteristic behaviors and physical features.  The inheritance pattern of fragile X is “X-linked” which means that women can be healthy carriers of the gene that causes fragile X.  Approximately 1 in 260 women are carriers of the gene for fragile X.

Currently, the American College of Medical Genetics recommends that carrier screening for fragile X syndrome be considered for the following individuals:
• Patients of either sex with mental retardation, developmental delay, or autism with or without physical or behavioral characteristics of fragile X, a family history of fragile X, or male or female relatives with undiagnosed mental retardation.
• Patients with a family history of fragile X or undiagnosed mental retardation.
• Fetuses of known carrier mothers.

For more information on fragile X syndrome, click here.