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Editor—Cystic fibrosis (CF) is an autosomal recessive disorder affecting 1/2500 live births in northern European populations1 and is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene.2 Data accumulated over the past 10 years have produced a remarkable repository of mutations and polymorphisms of the CFTRgene.3 However, the clinical consequences of many rare mutations are still poorly understood and functional studies are not routinely available. Because of the mutational heterogeneity and the rarity of many mutations, most clinical DNA laboratories offer tests that aim to detect 85-90% of CF alleles, and the systematic, cumbersome analysis of the rest of the gene is performed in selected cases only. With the increasing demand for prenatal screening of pregnancies from the general population, an increasing number of CF patients are diagnosed in utero in the absence of a family history of CF, either after preconceptional genetic screening of the parents, or after a routine fetal ultrasound showed a CF associated finding, for example, fetal bowel hyperechogenicity (FBH). In such instances, reliable genotype-phenotype correlations are required for appropriate counselling. FBH is defined as an echogenicity of a fetal bowel loop similar to or greater than the fetal iliac crest4 and is found in 0.5-1% of all second trimester pregnancies.5 Whether hyperechogenicity in FBH originates from the bowel wall or from intraluminal content remains a matter of debate.4 It is a non-specific sign that is associated with CF in some cases.1 4 6 In a study of 209 pregnancies with FBH, 68% had a subsequent normal outcome and 3.3% had CF, the other cases corresponding to chromosomal abnormalities (5%), gastrointestinal malformations (8%), fetal infection (4%), or unexplained fetal death (13%).5 7 Before 18 weeks' gestation, the assay of gastrointestinal enzymes in …