The fetus is not human being, how?

Okay I'll heard a lot pro choicer and feminist say the that a fetus is not a human being merely part of it's mother. Now while there is admittedly there are a lot of fairly reasonable arguments towards abortion rights it's seem to me that this statement is flying in the face of basic biology. A fetus has a complete set of chromosomes and by week 8 where most terminations take place has a clear heartbeat and a detectable brain activity. This is a living human by all scientific definitions is it not?