Abortion is terminating a developing life of a human being, which started from the formation of the zygote. That is why, abortion is a major sin, unless it is absolutely necessary to save the life of the mother if doctors decide that pregnancy under existing medical condition can kill the mother.
Abortion is not only a major sin, but also has a penalty like the penalty of killing and amount of the penalty depends on the age of the embryo.
Wassalam.