A hen will lay one egg every day until they have a ‘clutch’ of 12 eggs. They will then sit on them and ‘brood’ until

Question: A hen will lay one egg every day until they have a 'clutch' of 12 eggs. They will then sit on them and 'brood' until the eggs hatch 21 days later. Why 12? Is there some reason why hens will always wait for 12 eggs?
- Keywords:
  chicken,
  egg,
  hen,
  reproduction
Asked by michelletherezien to Charlotte, Jo, Kevin, Louise, Valeria on 19 Jun 2012.
- Joanna Cruden answered on 19 Jun 2012:
  
  I am not sure but I guess this may be the optimal number of eggs for her to get at least some healthy chicks from as some won’t survive

Question: A hen will lay one egg every day until they have a 'clutch' of 12 eggs. They will then sit on them and 'brood' until the eggs hatch 21 days later. Why 12? Is there some reason why hens will always wait for 12 eggs?