Millions of married couples see bringing up children as fulfilling and worthwhile. But for many couples the first real stress of marriage comes when the conventional picture of family life is complete; comes with the arrival of children. In this country we frequently have romantic illusions about children: ' they make the marriage ' we're told. But all too often they very nearly break it. We may expect our marriages to face economic uncertainty, emotional insecurity, sexual incompatibility, but some of us are unprepared for what happens when the children come.