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I ate a double-yolk egg for my lunch. If it had been fertilised, would two chicks have grown/hatched from one egg? Charlotte

Double-yolk eggs are fairly rare – it’s thought that about one in a thousand commercial hens’ eggs have two yolks, and multiple yolks (triple, quadruple or more) are even rarer.

They are a result of a malfunction in the chicken’s egg-laying process. Normally, an ovum is formed in the hen’s ovary and develops into the yolk. When it reaches the right size it is released into the oviduct. On its journey through the oviduct, the egg white (albumen) and the shell form around the yolk to create a whole egg, which is then laid. The whole process takes about 24 hours and the laying of one egg usually triggers the ovulation of the next.

Egg formation and laying are regulated by the hen’s hormones and occasionally things go wrong, especially in young hens. Two or more yolks may be released into the oviduct at the same time and become encased in one shell, resulting in a double-yolker.

Commercial eggs are unfertilised and do not develop into chicks. However, if domestic hens are kept with a cockerel and the eggs are fertilised then it’s possible that a double-yolker would contain two chick embryos.

There are a few very rare cases where ‘twin’ chicks have hatched from a single fertilised egg, usually with human assistance. Generally, though, the embryos from a fertilised double-yolked egg would not survive as there simply isn’t enough room in the egg for them to develop. Unlike mammals that have multiple embryos and a nice stretchy uterus, the eggshell cannot expand to accommodate more than one chick. Also, a chick has to be able to rotate within the egg so it can reach the air pocket in the round end, before pecking its way out. This would not be possible if two chicks were squeezed together into one shell. So fertilised double-yolkers are generally doomed but unfertilised ones make a great lunchtime bonus.