Reliable prenatal diagnosis of CF is possible using chorionic villus sampling (CVS) or using amniocentesis. It is preferable that DNA testing of the parents prior to a baby's conception is conducted to determine the CF mutations that they carry. This makes the results of prenatal tests easier to interpret.
Chorionic Villus Sampling
CVS is an invasive prenatal procedure and is performed between 11 and 14 weeks of pregnancy. A small piece of the developing placenta is removed and the DNA from this tissue is extracted and tested for the faulty CF genes. It is then possible to determine whether the fetus has CF. If the fetus is affected, there is the option of terminating the pregnancy. Alternatively, the parents may choose to continue with the pregnancy with the knowledge that the baby will be born with CF. The birth of this baby can then be planned with the necessary specialists in place. The CVS procedure carries a 2 to 3 % risk of miscarriage. The advantage of this procedure is that a result for CF could be obtained relatively early in the pregnancy so that pregnancy management can be discussed and organised timeously.
Amniocentesis is another prenatal procedure and that can be performed between 16 and 20 weeks of pregnancy. A small amount of amniotic fluid from around the fetus is removed. Cells in this fluid that originate from the fetus (containing fetal DNA) are tested for the faulty CF genes. Amniocentesis has a smaller risk for miscarriage than CVS (approximately 1%), but as this procedure is performed later in the pregnancy, decisions regarding continuation of the pregnancy, should the fetus be affected, may be more difficult.
Pre-implantation diagnosis (i.e. checking at the very earliest stage of embryonic development whether CF is present) is at present only available at some specialist centres. Eggs and sperm are harvested from prospective parents who have already had a child with CF. Following the procedure of in vitro fertilisation, the developing embryos are screened at a very early stage for their CF status. Selected embryos, free of CF, are implanted into the mother's womb to continue their development.
Parents of a CF child who are planning to have more children and who wish to have antenatal tests performed to see whether the fetus has CF should consult with their doctor and with a genetic counsellor/geneticist before embarking on a new pregnancy.