eli5 Eggs in the supermarket state on the box “Double Yolk”. Without cracking them, how do they know….?

Question

Apologies if this is ridiculous and should be in no stupid questions but it just struck me

In: 2748

Answer 1

I buy my eggs direct from a farmer and asked him last weekend!

Any hens under 14 weeks are guarenteed double yolks. It’s something to do with how fertile they are

Then from 14 – 20 weeks it xtra large eggs, that can sometimes be double yolks.

This week I ordered too late so had to get the extra large instead of double yolks. Apparently they were 16 weeks old – anyway – all were double yolks anyway