There's an old pro-abortion argument that has received a lot of attention on twitter lately. It goes something like this:
Imagine that you are in a burning building along with a five year old child and an incubator of human embryos. You know that you can save either the single child or the entire incubator full of embryos, but you cannot save both. Which one will you choose to save? If you chose to save the child, then you have proven that a child is more valuable than a human embryo which means that unborn humans do not have as much moral value as born humans.
There have been several excellent responses to this argument, but I'd like to point out the two responses that I think are the best of the lot.
"Give instruction to a wise man, and he will be yet wiser: teach a just man, and he will increase in learning." (Proverbs 9:9)