How do I ensure my baby's head stays the same shape?

Tummy time! Many babies develop flattening of the back of the head in the first 4-6 months from sleeping on their backs. Tummy time can help to avoid this. Also, make sure you put your baby in different seats, different positions and encourage him/her to look to both sides in the first few months. Most flattening resolves on its own when babies start to sit up (after 6 months).
Tummy time and chang. The head 'molds' or changes shape due to pressure from layind down and the head flattening on tje surface. Tummy time relieves that pressure. Also frequent position changes and movement while laying down will help.