A stranger needs a kidney transplant, or he will die. You are the only tissue match. Should you be forced to give up your kidney to save the stranger?
The attempted parallel fails on several levels:
1. A fetus in the womb isn't a stranger.
2. Pregnancy isn't some flukey thing that happens out of the blue; we know what causes it.
3. Pregnancy isn't permanent.
4. Abortion isn't letting somebody die who was moribund anyway; it's causing the violent death of somebody who would likely have lived a long life otherwise.
If we want to make the imaginary situation more akin to abortion, the scenario would have to go more like this:
You and your boyfriend/husband have a hobby that involves use of toxic chemicals that are fairly safe for adults, but can cause fatal kidney problems in children. Knowing this, you nevertheless pursue this hobby, exposing your child to the toxic chemicals. Your child ends up suffering kidney failure as a result. Should you be required to submit to a temporary transplant of one of your kidneys for the nine months it will take his kidneys to heal, or should it be perfectly legal and socially acceptable for you to hack him to death with a machete so you don't have to deal with any of this situation?
Does it look just a tad different now?