Congenital idiopathic dilatation of the common bile duct, called commonly and incorrectly congenital cyst of the common bile duct, is sometimes diagnosed during pregnancy. The latter may be the source of severe complications. The disease should thus be treated rapidly, even if the pregnancy is at a late stage. In a case of this type we carried out surgical treatment in two stages: firstly simple external drainage of the common bile duct and of the gall bladder and only after labour has been brought to term by Caesarian section, for breech presentation, was a cystojejunostomy carried out on a Y shaped jejunal loop.