@Ray:Very good,you have biology aspect of it explained very well.
Let me repeat something and add to it.

A fetus comes from the fertelization of an ovum,right?Your argument is that through killing that fetus,we deprive him from the right to live.But,if it wasn't for the entry of the sperm to the vagina of the women...the baby wouldn't have existed anyway.Meaning that if it wasn't for the "want" of the two people involved in this sexual act,the baby wouldn't have existed.In conclusion,if their was an actual "desire" for the baby to exist,then he would have existed.This case is more related to the case of "oops,I forgot to wear a condom!" in which mistakes happen.There are countless of other situations as well.

Another point is the negative effects which arises from not allowing Abortion which redundant covered pretty well.An increase in the amount of "unwanted babies" ratio will be really higher than the families who are will to adopt a kid anyway.