Unawareness of options and results. Many pregnant woman believe that it is a choice between abortion and keeping the baby, and others believe it's a choice between abortion, keeping the baby and closed adoption. The truth is there are many options including abortion, keeping the baby, temporary foster care, closed adoption, or open adoption. Many don't know what the results would be. Abortion and closed adoption can cause emotional suffering for the mother, of about equal amounts, and sometimes abortion can cause complications such as infertility and death. Keeping the baby can be hard in some situations, which is why foster care and adoption were designed. Open adoption, a little known phenomenon, is where the birth mother keeps contact with the child. It is quite recent in our culture, but many other cultures such as Vikings and Inuit have long had open adoption(although usually for different purposes). Temporary foster care is for the mother who will eventually be able to care for the child, but can't now. In cases where the child has a disability, it is quite common for the mother to have very little understanding of the disability, and prejudice is common among medical practitioners who are likely to give a very negative image of the condition(their view of disability is quite similar to eugenecists, ie, that disabled people are subhuman and better of not being born at all). An example I heard of was a woman who, upon hearing that her embryo had Down Syndrome, wondered if the child would ever walk(the average age for walking is between 2 and 3 for them). It's important for women in that position to meet children with the condition, and get accurate information.
Pressure from family, friends or other people. Many unmarried pregnant women are pressured to get rid of the baby, and threatened with consequences if they do not. Some feel as though getting pregnant is shameful and no one should ever know. Many of those with disabled fetuses believe that they 'don't have the right' to bring a disabled child into the world(Why are they viewed as less worthy? Hasn't our society decided that all people are of equal worth?).
Once people deal with those impediments to true choice, then we can debate the ethics of abortion. But to debate the ethics based on a fictional view of what the situation is is pointless. We have to understand the reality before we can argue about what should be done.