OK, first the science.
Homo sapiens is a species. Like all mammals, it has a haploid stage and a diploid stage to its life cycle. BOTH stages are *human*.
Our haploid stage is single celled and consists of our sperm and egg cells. They have half the number of chromosomes (23) as the diploid stage (which has 46 chromosomes, this is what defines haploid and diploid stages), which is multicellular.
Life does not begin at conception; it continues *through* conception. The haploid stage *is alive*. Human life (Homo sapiens) has been ongoing for the last 1-200,000 years.
So, sperm and egg cells are *human life*. They are also *individuals* in the sense that they are each genetically *unique*. The sperm and egg cells do NOT have the same DNA as the parents: they have half the chromosomes and each sperm and egg have a different mix in that half.
Next, every cell of the diploid stage has the same set of DNA. Liver cells, kidney cells, nerve cells, etc ALL have the same human DNA. A liver is alive, is human, and has a full DNA complement.
Conception (unification of sperm and egg cells) is not the same as becoming pregnant. To become pregnant, implantation has to occur. And implantation happens a couple of days after conception. Many conceptions don't implant: they are expelled with no resulting pregnancy.
No *new* DNA is formed as a result of conception. All that happens is that the chromosomes from the egg and sperm *together* constitute the chromosomes of the diploid stage. The 46 chromosomes constitute 23 pairs.
While the basic structures for many organs are laid out early in pregnancy, most do not actually start functioning until later. Most relevant for this discussion, the basic outline of the brain is laid down by the 6th week of pregnancy, but the interconnections between neurons that *define* brain activity don't appear until after the 23rd/24th week of pregnancy.
The developing embryo or fetus uses the resources of the individual carrying it in her uterus. It affects the immune system, the endocrine system, and puts a great deal of stress on the body.
It is also relevant to discuss a bit of what happens at the end of life. Death is *defined* by brain death: the cessation of brain activity at or above the brain stem. The neurons don't interact with other neurons and we call that brain death.
BUT, brain death does NOT mean the other organs of the body are dead. The brain is particularly susceptible to deprivation of oxygen and starts to die in a few minutes when the supply is stopped. Other organs, like liver, kidneys, heart, etc continue to live, sometimes for a quite extended period after brain death.
This is why we can do organ transplants and they work. Death is a process, not an event.
That is the science (well, a part of it).
Now, several questions arise in the *legal, philosophical, and moral* discussion about abortion.
One question is about individuality. When is a new individual formed? The scientific answer depends on how you define the term 'individual'. Does genetics define an individual? or does independent existence? or does consciousness? Science cannot answer these questions because science can't address how things *should* be defined. It can only take the definition given and attempt to supply the answer based on that definition. Which definition is to be used is a legal, philosophical, or moral question.
A few things are clear:
1. Whether something is human and alive is NOT the central question.
A liver or kidney are living human tissue. They have a full DNA complement and are diploid tissue. But we don't give them *moral* weight.
2. Whether something is a living human individual is NOT the central question.
Sperm and egg cells are living human individuals, but we don't have moral qualms when they die.
3. Whether abortion is right or wrong is NOT a scientific question. Science can't even answer the question of when a new individual begins without a definition of the term 'individual' and *that* is a philosophical question and not a scientific one.
Science can *inform* our debate. But it cannot answer moral or legal questions alone. Those require us to figure out our *values* and science cannot answer the question of which values to adopt.
So, the question is whether we value the developing embryo or fetus that is growing inside of an adult human more than the desires of the adult human it is growing in. And that is not a scientific question.