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Chance of CF for a fetus

Ι am in my 22nd week of pregnancy and the fetus was diagnosed with hyperechogenic bowel grade II. I did not have any bleeding during my pregnancy. After the recommendation of my gynecologist I did an amniocentesis and I and my partner gave a blood sample. The amniotic fluid and our blood was checked for 75% of CF mutations and all the results were negative. What are the chances of the baby having CF? Do you think we should do any further checking for a larger percentage of CF mutations, or the tests we already did provide adequate “safety”?
"Dear friend,
You are pregnant in the 22nd week and the fetus presented findings of a hyperechogenic bowel.
The hyperechogenic bowel may be have various causes, which I believe you have checked through the amniocentesis, such as chromosomal anomalies, congenital infections, cystic fibrosis, bleeding, while it is possible not to find any particular cause.
The chance of CF being the cause of the hyperechogenic bowel is 1% or slightly more. By testing both partners negative and a mutation detection rate of 70%, the residual risk of having a child with CF would be 1/38.168, respectively 0.00262% [Castellani et al. in the Journal of Cystic Fibrosis 2010, 9:165-178: “Benchmarks for Cystic Fibrosis carrier screening: A European consensus document.”] and is therefore 10 folds less than the "natural risk" of any couple without any testing and no history of CF in the family. By increasing the mutation detection rate to 90%, the residual risk of having a child with CF will go down to 1/357.143, respectively 0.00028% and is therefore 100 folds less than the "natural risk" of any couple.
Therefore you can see, that the residual risk is very, very low, even with 70% mutation detection rate, but of course could even be lowered by testing for 90% of mutations.
Sometimes, special findings or situations (like in your case the hyperechogeic bowel) give reason for being especially worried about a certain disease and parents would like to have the higest possible security, even if the chances with the already performed tests are very, very low.
Therefore, I would not be against you, or the fetus being checked for 95% of CF mutations, in order to decrease as much as possible the probability of CF, which, although decreased by the tests already performed, is not decreased to the lowest possible percentage.
If you wish, I am available to discuss your case. You can book an appointment at the CF Center at Agia Sofia Hospital, in order to see you in person, if you live near Athens. Otherwise, we can talk over the phone.
Yours friendly,
Dr. Stavros Doudounakis and Dr. Daniela d'Alquen