My cat recently gave birth to 5 kittens but the second one wasn t completely formed (sorry I don t know how to explain it). Why is that?

Sorry to hear that.

Deformities occur for two reasons. Genetics, where the kitten has inherited a trait or mutation, or a birth-defect caused by a problem in the womb whilst the embryo was developing.

If you're looking for more detailed information, the Messybeast web site medical anomalies reference pages may help.

http://messybeast.com/curiosity-index.htm

The food you're giving to her contains teratogens?
The tomcat responsible for those kittens is closely related, genetically to your queen?
Stress!

This is often a hidden genetic issue. The fetus just does not develop properly in the womb. Be aware that the kittens may have inherited something that isn't showing up yet. And this is Nature's way of telling you that you need to get your cat spayed. Unformed fetuses and other stillborns often cannot be passed, leaving you with a very expensive emergency vet bill for a c-section delivery.

it was either a runt or a stillborn