The Bible is silent about abortion in the same way it is silent about the Trinity.
In other words, the word itself is not there, but the teaching is.
First of all, Scripture is clear from beginning to end about the fact that human life is created by God, is in God's image, belongs to God, and has a destiny with God. The obligation to respect human life is illustrated in a thousand ways. Our obligations to each other are crystal clear. See our pamphlet Scripture's Teaching Against Abortion as well as our outline of Scriptural themes for more details.
But there is another consideration here.
Suppose those who criticize us by claiming the Bible is silent on abortion were right. Suppose there were nothing in it to indicate that abortion is wrong.
Am I then to consent to the killing of little children?
Many of those who make this argument do not believe in the Bible anyway. They make all sorts of judgments about what to do and not to do in life independently of what is in the Bible.
Are they then to say we cannot do the same?
I believe in the Bible. And it condemns the killing of the innocent, which abortion is.
But even if the Bible were silent about abortion, I have enough decency and good sense to see that abortion is an evil that neither I nor anyone else should tolerate.