The human dignity is a very important issue. It is one of the most important feature of human nature. As a Catholic, I believe and I am convinced that the human dignity comes from God.
We have our dignity because we were created as a picture of God – “God created man in the image of himself, in the image of God he created him, male and female he created them” (Genesis 1,27). The Old Testament teaching about human being created as picture of God was developed in the New testament, where Christ added that a man and a woman are not only the picture of God but we are the God’s children – “…so that you may be children of your Father in heaven…” (Matthew 5,45) or “You must therefore be perfect, just as your heavenly Father is perfect (Matthew 5,48). Human being as a picture of God and as God’s child is a center of entire creation and his or her dignity comes from that.
The Bible teaching is repeated in many Church documents. It is hard to listed them all. The matter of human dignity is so important and is often present Church’s teaching. There are among others: the Declaration of the Vaticanum II – Dignitatis Humanae from 1965 or the Pope Leo XIII’s encyclical - Rerum Novarum (On the Condition of Labor) from 1891. In the context of this question, it is appropriate to name also the John Paul II’s encyclical - Evangelium Vitae from 1995. Of course, there so many other Catholic Church documents which talked about the issue of human dignity.
In my opinion we should not talk about the beginning and the end of human dignity. God is endless and therefore if our human dignity comes from Him, it is also endless.
The human dignity means that human being comes from God. The God is at the beginning and at the end of human life. Therefore only God can make the decision when the human life is started and when is done. The abortion or euthanasia is contrary to the proper understanding of human dignity. The human being is a center and therefore nobody has the right to kill a man or a women. No matter what age they are in. The abortion or euthanasia are nothing else like taking out (killing) the human life.
In order to have good understanding of this matter I strongly recommended to read the above-mentioned encyclical of JPII - Evangelium Vitae (The Gospel of Life). This document talked about the value of human life based on the value of human dignity. There are also some important fragments regarding the abortion (Evangelium Vitae, §§ 58, 61-62) and the euthanasia (Evangelium Vitae, § 65).