Cystic fibrosis is a chronic condition caused by a mutation in the cystic fibrosis transmembrane regulator (CFTR) gene that disrupts the workings of chloride channels in cells. Children born to people with CF, or those carrying the CFTR mutation, can inherit the condition.
To develop CF, a child must inherit a faulty copy of the CFTR gene from each parent; it runs in an autosomal recessive pattern, meaning both parents must have at least one CFTR gene mutation. Their children have a 25 percent chance of inheriting a faulty gene and developing CF, and a 50 percent chance of being a carrier like the parents. (About 180 disease-causing mutations are known today.)
A prenatal diagnostic test can help identify, before birth, whether a fetus is carrying the CFTR mutation. Test results have a high degree of certainty about the CF status of the fetus, but cannot predict disease severity.
Types of prenatal diagnostic tests
Two types of prenatal diagnostic tests can be performed depending on the stage of the pregnancy.
Chorionic villus sampling (CVS)
Chorionic villus sampling (CVS) is done between 10 and 13 weeks of pregnancy. A small piece of tissue from the placenta is used to detect genetic abnormalities. The placenta grows along with the fetus, supplying nutrients to the child and removing waste products from its blood.
There are two ways in which CVS can be performed: transabdominal or through the belly, and transcervical or through the cervix.
In transabdominal CVS, the doctor passes a thin needle into the womb through the belly to obtain a small sample of the placenta. An ultrasound is used to guide the needle.
In transcervical CVS, the placenta sample is obtained using a small tube that is passed through the vagina into the cervix with the guidance of an ultrasound.
The placenta tissue is then sent to a laboratory for testing. Results can be expected within five to 10 days.
Amniocentesis can be performed at 15 to 20 weeks of pregnancy. It tests the amniotic fluid around the fetus in the womb. The amniotic fluid contains fetal cells that can be used to identify mutations linked to diseases such as CF.
For this procedure, the doctor guides a thin needle with the help of ultrasound through the belly and into the amniotic sac to remove a small amount of the amniotic fluid.
The sample is sent to a laboratory for testing. It may take two to three weeks for the results to become available.
Risks associated with amniocentesis include amniotic fluid leakage, infection, transmission of infection from mother to child, Rh sensitization, and needle injury to the fetus.
After the procedure
Both tests are invasive and may cause distress to the mother and the fetus. Their heart rates and vital signs will be monitored for a few hours after the testing to ensure they are both stable.
Blood tests performed earlier in the pregnancy can help determine the mother’s Rh type and predict the risk of Rh sensitization, or a mixing of Rh-negative and Rh-positive blood between a woman and her unborn child. The mother may be given Rho(D) immunoglobulin to prevent her Rh-negative antibodies from reacting with the Rh-positive fetal blood cells, or vice versa.
Vigorous activity should be avoided for a day or two after the testing.
Slight cramping and spotting are to be expected for a few hours but if it worsens the doctors must be informed.
The doctor must be immediately notified if there is vaginal bleeding, amniotic fluid leakage, unusual fetal activity, soreness and inflammation around the needle site, or fever.
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