There are a lot of reasons but it comes down to this: no one has a right to force a woman into an unwanted pregnancy, control her body or make life changing choices for her. If you feel so strongly about it - offer to have the unwanted fetus implanted in you, or start a compaign to force the father to carry it and see how you feel about having someone who neither knows you nor cares take control of our body.
A handful of cells is nothing more than a potential baby - it has no brain, no awareness, no consciousness, no feeling.
You are about 30 years behind the science on this issue, unless you're only talking about abortion done within 1 week of pregnancy.
And, FYI, it's a biological fact that the baby is not the mother's "body." It's not "her body." The baby is a separate body in her body; he or she is not like her kidney or liver or heart.
No, I'm pretty up to date on science.
Week 1 - egg is released
Sex occurs and the sperm goes looking.
At this point - the Morning After Pill can be used to prevent pregnancy (up to 5 days post sex) - however, pro-lifers seem to consider this an abortion so I imagine that you would consider the woman homocidal at this point.
Week 2 - 3 Conception. Sperm penetrates egg and becomes a zygote. One cell. Zygote contains all the genetic information to become a baby. Potential.
It spends a week traveling, becomes a microscopic ball of cells called a Blastocyst, and embeds in the uterus if it's lucky.
75% chance of miscarriage occurs.
This is where IUD controception works - it prevents the Blastocyst from embedding. I suppose she is a serial killer now.
Week 4 - Blastocyst embeds and starts to multiply. It's still just a ball of cells. 30% chance of misscarriage.
Week 5 - it becomes an Embryo. Cells start to differentiate. It doesn't even look remotely human.
Week 7 - arm and leg buds develop, further differentiation, primitive organs, size of a lentil,
33% of abortions take place before this time.
Weeks 8 - organs continue to develop and grow. The embryo is the size of a kidney bean.
At this point if a woman is going to have an abortion, 63 % have done so.
Only 4.8 percent of all abortions occur after 15 weeks.
Brain waves don't begin to appear as
intermittent bursts until 20 weeks and aren't consistent and synchronized until 27 weeks. This also means that it can not feel pain until this point.
It's a biological fact that it is not viable and entirely dependent on the mother's body for existence. It does not even have a functioning brain. Brainwaves define the existence of independent life.