It is a radical operation to get your head around.
Surgeons tell you they need to remove the entire top of your baby's skull, cut it into pieces and put it back together like a Meccano set, to change the shape of your child's head and give the brain enough room to grow.
But Alexandra and Jason Colston-Ing had few other options when the plates of their son's skull fused too early for his developing brain.
Like clasped hands, the fingers — in this case, the plates of the skull — should be able to able to move apart as the child's brain rapidly grows in the first few years of life.
But the main connective tissue running the length of William's head had joined, to create a longer and narrower skull with a pronounced forehead and the crest too forward.
The doting new parents hadn't noticed.
It was a happy snap of four-month-old William with his four-year-old sister, Zoe, which Mrs Colston-Ing sent her mother and sister-in-law that sparked the response from each, separately: "There's something not right with William's head."
Scans confirmed he had scaphocephaly, the most common type of craniosynosis.
Royal Children's Hospital plastic and reconstructive surgeon Jonathan Burge performed a total cranial vault, which about 75 kids undergo at the RCH each year.
Once in the operating theatre, with the scalp off to expose the 11-month-old's skull, Mr Burge marks the intended incision points with Texta.
Neurosurgeon Juliet Clayton will cut the skull at these points into five pieces, while Mr Burge alters the pieces to their desired shape to fit their jigsaw puzzle.
The new rearrangement of skull will create more of a teardrop-shaped skull rather than an oblong.
William has been home since Saturday, three days after the operation. The difference was instant to this parents when they met him in the recovery bay after surgery.
Mrs Colston-Ing said she now felt relief her boy's brain had enough room to grow and develop.
"The next chapter has begun," she said.