To what extent is unborn less valuable than a born life?
Choose two desert islands. Put 1000 fetuses on one and 1000 20 year-old women on another. Which group has the better chance of surviving and prospering?
An adult woman can immediately participate in a functioning society. A fetus requires a huge investment of time and energy before it can do the the same.
An adult woman has years of memories, skills, relationships, etc., which will be destroyed upon her death. The fetus has none of these.
Obviously, the human species needs some fetuses to come to term in order to produce a new generation. However, that is not a problem at this point in time. The problem is that the Earth is becoming overcrowded. So if a woman doesn't want to bring a fetus to term, there is no good reason she should.