Kiwi

The Kiwi bird lays an egg that can weigh up to a quarter of its body mass and hatches into a more or less completely developed adult kiwi. Kiwis lay the biggest egg in proportion to its size of any bird in the world even though the kiwi is about the size of a domestic chicken and it lays eggs that are about six times the size of a chicken's egg. The egg displaces most of the mother’s internal organs, stretches her ribcage and makes her barely able to move, eat, or even breathe.