With respect to those meanings of "human" that are relevant to the morality of abortion, any fetus is less human than an adult pig.Well, perhaps Dawkins intended to shock, but what he really accomplished was to make himself sound like a dunderhead.
For example, a fetus is fully human, a pig is not at all human. It's a simple matter of biological fact which any tenth grade biology student could understand let alone a professional biologist of Dawkins' stature.
Secondly, as an atheist Dawkins piggy-backs (pardon the pun) on theism by referring to moral considerations in the matter of abortion. This is a favorite tactic of atheists, one which they find it expedient to resort because on atheism there simply is no objective moral value. Lacking the resources to enable them to use moral language they employ the intellectual equivalent of sneaking past the turnstile by squeezing close to the paying customer and hoping that no one will notice.
The fact is that on atheism abortion is neither moral nor immoral, any more than a tonsilectomy is moral or immoral. Morality in a world without a transcendent moral standard is simply a matter of whatever you can get away with, a point that I try to drive home in my book In the Absence of God (Have I mentioned that I've written a book?), and abortion is something that, as matters stand today, anyone can get away with.
Thirdly, to place less value on a human fetus than on a pig is to open wide the gate to thinking that just as we can farm and slaughter pigs without violating any moral principle, so, too, can we farm and slaughter humans with moral impunity.
It's a frightening world that the modern atheist would lead us into, one where humans are no different than other animals, where humans have no particular dignity or moral status, where there's nothing special about us at all. That's, of course, the same sort of thinking that led to the 19th century slave markets and later to the 20th century horrors at Buchenwald and Auschwitz, and it's the ineluctable consequence of the atheist's denial that man is not a mere animal but is made in the image of God.