Article 6 of the Universal Declaration of Human Rights says, “Everyone has the right to recognition everywhere as a person before the law.”
The word “law” is used nine times in the UDHR. That is an indication of how important law is in our lives. Law has been part of our culture since the beginning of human civilization of the last 10,000 years, and since the precursors to law extending back through all of human society that preceded civilization, for the more than 100,000 years that people lived as hunters and gatherers before they began to live in cities.
Jeremiah 31:33 in the Bible says, “This is the covenant I will make with the people of Israel after that time,” declares the LORD. “I will put my law in their minds and write it on their hearts. I will be their God, and they will be my people.”
According to Wikipedia, the “Code of Hammurabi is a well-preserved Babylonian code of law in ancient Mesopotamia, dated back to about 1,754 BC (Middle Chronology). It is one of the oldest deciphered writings of significant length in the world.”
Law is simply talking about rules. Families have rules. Families have been part of humanity since we became humans. Humans are unique in the amount of time that is needed for offspring to develop after they are born, before they have developed to adulthood. Families need rules. Rule number one, everyone contributes to the welfare of the family to the best of their ability from three years old on up. That was essential for hunters and gatherers.
Today we have civilization. The first cities began about 10,000 years ago. Jericho was one of them. For people living together in larger numbers, it was necessary to codify the rules into laws.
Today we recognize the thirty articles of the Universal Declaration of Human Rights as part of supreme law for all of humanity, along with the principles of the Earth Charter. The principles in those two documents provide a fairly complete set of principles with which we can govern ourselves.
I recognize the lack of the mention of abortion in either the UDHR or the Earth Charter. Abortion is a question for which there simply is not agreement on the principle, and so it is not possible to have a law about it. The law rests on some principle.
Part of the difficulty with the question of abortion is the related question about the existence of a soul. Introducing the question of the soul is an expression of dualism, the idea that there is something fundamentally different between material reality and spirit. An alternative viewpoint is that everything is connected, and that we are One. It would be a violation of Article 18 of the UDHR to force anyone to accept either of those viewpoints.
Consciousness seems to be expanding. Some level of consciousness may be necessary in order to suffer. Carl Sagan observed that the capacity for human consciousness began to develop in the third trimester before birth, when the neurons in the brain began to connect with each other at an enormous rate. The number of humans is still expanding at a rate of over 80 million people per year, with a current total of about 7.6 billion people.
The solution is to expand the degrees of freedom  of every person who is capable of reproduction. As we succeed with that, it is likely that the rate of births will decline to approximate the rate of deaths, so the global population will stabilize below eight billion people. This will expand the degrees of freedom for everyone.
 S. Cerezo, G. Ballester. Fractal AI: A Fragile Theory of Intelligence. URL https://arxiv.org/ftp/arxiv/papers/1803/1803.05049.pdf
August 3, 2018