Egg production in the hen is controlled by the release of specific hormones, which in turn stimulate the release of a single egg yolk from the ovary. After the yolk is released from the ovary it continues its journey through the hens reproductive tract where it develops into an egg with a hard outer shell. Usually, only one yolk is released by the ovary in a given day. However, sometimes two egg yolks or on rare occasions, even three yolks may be released at the same time resulting in the formation of a double or triple yolked egg.
This release of more than one yolk at a time is due to an over stimulated ovary which occurs as a direct result of the increased level of reproductive hormones in the hen. This phenomenon appears more commonly in young hens and is also seen more frequently in meat-type strains of hens verses egg-type hens. Genetics may also be a factor involved with some hens naturally producing a higher percentage of double yolked eggs than others.