You were once a single cell. Every one of the 100 trillion cells in your body today is a direct descendant of that zygote, the primordial cell formed by the union of mother's egg and father's sperm. Each one is genetically identical (allowing for copying errors and environmental damage along the way) to that cell. Therefore, if we scraped a cell from, say, the inner lining of your cheek, its DNA would be the same DNA that, years ago in the original zygote, contained the entire plan for creating you and every part of you.
Here is the mystery: Why can the zygote, as it multiplies, produce every different kind of cell in the body- kidney, liver, brain, skin- while the skin cell is destined, however many times it multiplies, to remain skin forever? As the embryo matures, cells become specialized and lose their flexibility and plasticity. Once an adult cell has specialized- differentiated, in scientific lingo- it is stuck forever in that specialty. Skin is skin; kidney is kidney.- Charles Krauthammer, in Essay: On the Ethics of Embryonic Research, from his book Things That Matter
Many times I have heard pro-choice supporters say that the pro-life side is lying when it says that human life begins at conception. Here, in the words of a medical doctor/psychiatrist is the scientific truth. Why do they persist in denying it?
As for those crazed pro-choicers who state that every sperm is a life and what happens to those millions of sperm that are wasted, what possible answer can you give? They are worse than ignorant; they haven't even begun to think about these questions, yet they are quick to spout their arguments for abortion.
The same goes for those who say that a woman aborts an egg every month. What planet are they living on? Perhaps they need to spend a week on a chicken farm and see that chickens lay eggs every day, but those eggs are not fertilized and do not become chickens, they cannot become chickens. That would take the visit of a rooster to make that change occur.
The biological knowledge of pro-choicers is juvenile in so many cases. I would challenge them to really do some reading and find out about the real facts of the birds and the bees. They might just be surprised by the science.
People who scratch the surface of science tend to be atheists; those who plumb its depths tend to become believers. Knowledge brings humility.