You don't have to weigh it against the woman's suffering if you acknowledge the fetus is innocent in this situation. It didn't ask to be put there. Actions taken by others put it there.
Who is at fault for the situation matters.
Being cruel to the innocent to protect the interests of some of those responsible for putting the innocent fetus in that situation in the first place... does that sound ethical? Does that sound like justice? Those who think we should weigh the suffering of the fetus against the suffering of the woman apparently seem to think so.
Everyone agrees its best to avoid unwanted pregnancies in the first place. People who are pro-choice are strongly in favor of sex education and access to contraceptives. Empowering women to have control over their reproductive health is the best way to prevent unwanted pregnancies.
But expecting that to prevent all unwanted pregnancies isn't realistic. Even a woman who always uses contraceptives can get pregnant. Should her interests and her suffering be dismissed as well? What about the family struggling to provide for the children they already have? Should those kids grow up getting less attention and resources from their parents?
In my mid 20s my girlfriend stopped taking her birth control without telling me and ended up pregnant. We talked it over and she got an abortion. Should I have been forced to father a child I didn't want and wasn't ready for?
What about women who are raped and children who are molested?
Pro lifers aren't interested in weighing the suffering and hardship resulting from forced birth against anything because they aren't trying to resolve a moral dilemma. They're trying to force their absolutist views on the rest of society.