I had the interesting experience of reading P.D. James's novel The Children of Men at the same time as I was reading Peter Singer's book Writings on an Ethical Life. Singer justifes infanticide with breathtaking ease, arguing that infants are "replaceable." I don't want to be unfair to his position; it is not as if he thinks it is simply ok to go around killing babies. But he is clear: if you do not want to care for a Down's syndrome child (because it will take away some of your happiness) it is ok in his ethical view to abort that child or to euthanize him. Then, just go on ahead and have the "normal" child you prefer to have. It is a replacement for the other.
Infants are not replaceable. In James's novel, she puts some pressure on Singer's position by imagining a world in which suddenly, inexplicably, humans are unable to have babies. The year of the last child is named "Omega." 25 years go by; soon humanity gives up hope on life. The novel makes you think again, and not at all sentimentally, about how precious human life really is. We simply take fertility for granted. The protagonist in the novel, Theo, learns to come outside of himself to fight for others. It is this kind of change in our way of thinking about others that will save us. Children are not primarily for our happiness (though certainly they give joy). They are the gift of God because life itself, in all its variety, is the gift of God.