Abortion is a type of killing of a human being under formation, that is why it is a major sin. The Penalty of such major sin is called Diyah which is obligatory on the person who did the abortion as well as a Kaffara of killing . The Kaffara is three parts; freeing a slave, fasting two months continuously and feeding sixty poor Momin's. The amount of the Diyah depends on the number of the days of the embryo since the formation of the zygote. 
'Wassalam.