Actually, I believe that the church does not make this distinction. Forcing the delivery of a child that will not be sufficiently developed to survive is not considered any different than aborting the child. I will try to find a link for this but I remember seeing this very recently.
http://www.newadvent.org/cathen/01046b.htm
As to the time, let the questioner remember that no acceleration of birth is licit unless it be done at a time, and in ways in which, according to the usual course of things, the life of the mother and the child be provided for".
The Church will allow a premature delivery in order to baptise the child if it is medically and morally certain that the child will otherwise die prior to birth.
Otherwise, you are right, prematurely induced labor is condemned.