The Catholic Church believes that all human life begins at the moment of conception, and from that point has potential. This means Catholics believe that all life is sacred from the moment of conception. This is otherwise known as the sanctity of life.
As a result, Catholics believe that no human should interfere with or interrupt what nature has planned.
Before I formed you in the womb I knew you...Jeremiah 1:5
You created every part of me; you put me together in my mother’s womb. When my bones were being formed carefully put together in my mother’s womb, when I was growing there in secret, you knew that I was there – you saw me before I was born.Psalm 139:13, 15-16
These teachings also make the point that all life is God given. God is the one who creates all life and therefore all life should be treated as that, namely a gift to be cherished and cared for.
As Catholics believe that life is God given, they also believe that no one other than God has the right to take or end life. As they believe that life begins at conception, this includes both abortion and euthanasia.