Antenatal diagnosis of lethal Costello syndrome: how fetal exome sequencing using NHS England's R21 pathway accelerated the diagnosis of non-immune hydrops and improved patient experience.

Costello syndrome is a very rare genetic condition with a poor prognosis, caused by a mutation in the HRAS gene. We report two lethal cases diagnosed antenatally using recently introduced exome sequencing technology (R21). Without this testing, they are unlikely to have been diagnosed so promptly, which the families found very helpful in coping with their loss.