Cystic Fibrosis Prenatal Screening

Cystic fibrosis (CF) is a chronic, progressive and frequently fatal genetic disease of the body's mucus glands that often causes severe respiratory and digestive disorder. In the United States, it affects about 30,000 children and adults. The disease causes the body to produce an abnormally thick, sticky mucus that obstructs the pancreas, preventing enzymes from reaching the intestines to digest food. The sweat glands and the reproductive system also are usually involved.

Caucasians of Northern European ancestry have the greatest chance of being carriers of the cystic fibrosis gene. With improved treatments, the lifespan of individuals with cystic fibrosis has increased so that on average it now is about 30 years.

Cystic fibrosis has a variety of symptoms including very salty-tasting skin; persistent coughing, wheezing or pneumonia; excessive appetite but poor weight gain; and bulky stools. The sweat test is the standard diagnostic test for cystic fibrosis. This simple and painless test measures the amount of salt in the sweat. A high salt level can indicate that a person has CF.

People who carry traits for these disorders may not have symptoms. However, if both parents are carriers of the same abnormal gene, the chance of having an affected child is 1 in 4, or 25 percent, for each pregnancy. A special blood test, called a carrier test, may be able to tell if you carry such a trait.

Screening

Carrier testing, which is performed by a blood test, can detect mutations in the cystic fibrosis gene. There are over 800 genetic changes or mutations that are known to cause CF. Some CF carriers have rare mutations, which will be missed by the test. Genetic testing for carrying the gene detects 85 percent to 90 percent of the gene changes known to cause CF among Caucasians. Genetic testing identifies a higher percentage — about 95 percent — of carriers in the Ashkenazi Jewish population, but fewer carriers in other ethnic groups. Screening is also available for other ethnic groups but is able to identify fewer carriers because many mutations have not yet been identified.

A negative carrier test does not eliminate the chance you may carry the CF gene, but can lower that chance. Some people have rare CF mutations that cannot be detected by the routine testing available. There is a small chance that someone with a negative test could be a carrier and have a child with CF.

Diagnostic Test: Chorionic Villus Sampling

Chorionic villus sampling (CVS) can detect chromosomal problems such as Down syndrome as well as other genetic diseases such as cystic fibrosis, Tay-Sachs disease and sickle cell disease in at-risk fetuses. An advantage of CVS over amniocentesis is that it is done much earlier in pregnancy, at 10 to 12 weeks, rather than at 15 to 20 weeks.

The CVS procedure involves removing a tiny piece of tissue from the placenta. Under ultrasound guidance, the tissue is obtained either with a needle through the abdomen or a catheter inserted through the cervix. The tissue is then cultured and a karyotype analysis - an analysis of the chromosomal make-up of the cells -- is performed. It takes about two weeks to receive the results. Because the test can be performed much earlier in pregnancy, results are typically available by the end of the third month. There is a small risk of miscarriage as a result of CVS — 1 in 100 or less. Miscarriage rates for procedures performed at UCSF Medical Center are less than 1 in 200.

Treatment

If you have positive results on a screening test, we recommend that you discuss this with your own doctor and a genetic counselor. Options for further diagnostic testing will be explained. The decision as to whether to have invasive genetic testing is up to you.

If a diagnostic test finds a genetic abnormality, the significance of such results should be discussed with experts familiar with the condition, including a medical geneticist and a genetic counselor, as well as your own doctor. Medical geneticists and genetic counselors are available at the UCSF Prenatal Diagnosis Center. Referrals and support information are available for all decisions.

Reviewed by health care specialists at UCSF Medical Center.
Last updated February 2, 2012

This information is for educational purposes only and is not intended to replace the advice of your doctor or health care provider. We encourage you to discuss with your doctor any questions or concerns you may have.