@InYourFaceNYer Abort it and try again. It would be immoral to bring it into the world if you have the choice.
— Richard Dawkins (@RichardDawkins) August 20, 2014
There is no need to recount the entire thing since the story is widely available (here is one source). So I will only focus on the content of his tweet.
First of all, you cannot abort someone who has not already been brought into this world. Conception produces a new, genetically distinct human individual who has a body of his very own. When one commits abortion, it is this individual who is killed. This individual already exists. Birth is only one event in the life on an individual, and biologically speaking, that event is less significant to one's body than adolescence.
Second, as an atheist, Dawkins has no basis upon which to suggest that any action is "immoral." Sure, he can say that something is good or bad, but morality requires an objective moral law and a source for that moral law, and Dawkins believes that no such thing actually exists.
Dawkins has also stated long ago that killing babies after they are born is acceptable, too. At least that's consistent with his religious worldview. It's insane, but consistent. Here's a good response by scientist Jonathan Sarfati.