My son is 21 months and has congenital hydrocephalus. at what stage do they decide to put a shunt? the latest mri scan shows that he has a cyst that has grown from 6cm at birth to about 12.9cm now.

Depends: This depends on the type and effect of the process. The CSF fluid producing process can make 1 tablespoonful of fluid/hr. If the head is expanding rapidly because there is total blockage they would intervene quickly. If not & the kid is otherwise thriving they may feel the risk of both the procedure & the presence of the shunt is more risk than leaving it alone. I would discuss this with his doc.