I was once told that a fetus is special because "it is something new."
That is, something completely unique that will never be duplicated. It is one permutation out of trillions and trillions of past, present, and future possibilities with its own opportunity to contribute to human history.
The fetus embodies humanity's future, and carries in its DNA the virtually unlimited potential of our species. To stamp out that unpredictable spark of life is to deny yourself a chance to affect that unseen future. It's easy to make a dark, dismal prophecy of what's to come, and to fulfill it by ending the life in the womb, but what if it's wrong? No one knows what's in store, so we all need to invest in our future rather than turn our backs on it.