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Prenatal diagnosis

Question
Hello, we are a couple desiring a child. I'm Breton [inhabitant of Brittany, a region in France] and my husband has a cousin who has CF. Is it necessary to make an ante-natal or prenatal screening or a specific follow-up? Thank you.
Answer
Hello,
Your question comes down to whether you as a couple are at risk of giving birth to a CF child.
The frequency of cystic fibrosis, precisely known since the implementation of neonatal screening, is approximately 1 in 4600 births in France and 1 in 3000 births in Brittany. In the absence of a CF family history or result of a molecular genetic analysis, the probability of carrying a CFTR gene mutation causing cystic fibrosis is 1/34 in the general population in France and 1/27 in Brittany (your case).
The probability for your husband to be carrier of one of his CF cousin’s 2 CFTR mutations is 1/4. The probability for your couple to be both healthy carriers is 1/108 (1/27 X 1/4) and in this case the probability for your couple to give birth to a CF child is, at each pregnancy, 1/432 (1/108 x 1/4), ie 10 times higher than that of a couple without any CF family history.
You can accurate this probability by performing, you and your husband, a search for CFTR mutations. This test says "heterozygous" (or healthy carrier) screening laboratory requires a sample of a few drops of blood to be analyzed in a molecular genetics laboratory.
If no member of the couple is carrying a CFTR mutation, the probability of passing the CF mutated gene to their children is almost zero (even in this case there is still indeed a very small risk because few mutations have not been identified so far).
If one member of the couple (either father or mother) is a healthy carrier of a CFTR mutation, the probability of transmitting the disease remains almost zero but the probability of transmitting to the child the CFTR mutation of the "healthy carrier" parent is 1/2 at each pregnancy of the couple.
If both members of the couple are healthy carriers of a CFTR mutation, the probability at each pregnancy:
- to have a sick child is 1/4;
- to have a healthy carrier child is 1/2;
- to have neither a sick nor a healthy carrier child is 1/4.
In case of pregnancy of such a couple, it is possible to make a prenatal diagnosis of cystic fibrosis by chorionic villus sampling (fetal part of the placenta) at 11-12 weeks of gestation to study the DNA of the fetus (such a diagnosis is only possible if both mutations of the healthy carriers have been previously identified).
If the fetus is CF, a medical termination of pregnancy may be considered depending on the personal choice of the couple.
According to the French government decree of May27th 2013, these healthy carriers screening or prenatal diagnosis must be carried out "in the context of an individual consultation with a doctor working in a multidisciplinary team including clinical and genetic expertise". You can get in touch with such a team by contacting:
- the CF Center where your husband’s cousin is followed up (if you know it);
- the medical genetics department of a university hospital;
- or your general practitioner who will guide you according to your living place.
Hope that answers your question.
Gilles RAULT
with the kind contribution of Marie-Pierre AUDREZET, Laboratory of Molecular Genetics, University Hospital of Brest.
30.07.2013