Doctors are not sure what causes de Quervain syndrome. Patients often describe a feeling of inflammation, but studies have shown that the process is not inflammatory. People of all ages get it. When new mothers develop de Quervain syndrome, it typically appears 4 to 6 weeks after delivery. The old theory that it was caused by wringing out cloth diapers has been replaced by concerns about holding the baby, but changes in hormones and swelling seem more probable.