Description from the website:

This little body warmer has been made by special request from some of the staff on the neo natal units. It has buttons on the shoulders and opens flat so that the nurses can dress baby with the minimum disturbance. The original was knit in 4ply and measures approx 13” round the chest. It can be made smaller or larger by using larger needles and DK wool or smaller needles and 3 ply wool. As always check with your nearest Neonatal unit to see what their requirements are.

The pattern has instructions for either fingering (4-ply) or sport (5-ply).