Superfetation occurs when about 10 percent of cats come into heat when pregnant. This results in a second set of eggs being fertilised before the first litter is born. It is a result of the breeding capabilities of the domestic cat.
Normally pregnancy suppresses the sexual physiology of the female. However, female cats sometimes break the norms. There is another phase of sexual receptivity during the pregnancy cycle when there is a low level of pregnancy hormone.
The additional period of heat occurs around 3 or 6 weeks into pregnancy which lasts 9 weeks. This period of heat during pregnancy usually occurs between the 21st and 24th day. If the female mates again and her eggs are fertilised she’ll carry two litters at different stages of development.
The unborn kittens develop along side each other at different stages. When the female gives birth to the older kittens she may also give birth to the younger ones prematurely causing their death. If the younger ones go on to full term the mother will end up with too many kittens to fed as she’ll have a shortage of nipples 🙂 The mother normally finds a way to cope.
P.S. If your cat has experienced feline superfetation please share in a comment. Thanks.