That's a largely unanswerable question. What defines "human"? Is it the sequence of DNA? Is it the presence of a soul? Is it some level of thought? The first is knowable. The second is absolutely not knowable. The third is a bit squishy - is thought just synapses firing, or does a person have to articulate that thought into language (or something else)?

To me, it's viability, which is itself fuzzy and gradually getting to be a smaller window. Carry a fetus long enough that it's viable, you should carry it all the way (newly arising complications as an exception). Before that, it's not human enough to outweigh the costs/risks/etc to the mother.