Here's some info that hopefully will be helpful to anyone who is confused about when the life of a human begins.
From Libertarians for Life. I'm going to highlight the important parts....
Basic human embryological facts
To begin with, scientifically something very radical occurs between the processes of gametogenesis and fertilization — the change from a simple part of one human being (i.e., a sperm) and a simple part of another human being (i.e., an oocyte — usually referred to as an "ovum" or "egg"), which simply possess "human life", to a new, genetically unique, newly existing, individual, whole living human being. (an embryonic single-cell human zygote). That is, upon fertilization, parts of human beings have actually been transformed into something very different from what they were before; they have been changed into a single, whole human being. During the process of fertilization, the sperm and the oocyte cease to exist as such, and a new human being is produced.
To understand this, it should be remembered that each kind of living organism has a specific number and quality of chromosomes that are characteristic for each member of a species. (The number can vary only slightly if the organism is to survive.) For example, the characteristic number of chromosomes for a member of the human species is 46 (plus or minus, e.g., in human beings with Down's or Turner's syndromes). Every somatic (or, body) cell in a human being has this characteristic number of chromosomes. Even the early germ cells contain 46 chromosomes; it is only their most mature forms — the sex gametes, or sperms and oocytes — which will later contain only 23 chromosomes.1 Sperms and oocytes are derived from primitive germ cells in the developing fetus by means of the process known as "gametogenesis." Because each germ cell normally has 46 chromosomes, the process of "fertilization" can not take place until the total number of chromosomes in each germ are cut in half. This is necessary so that after their fusion at fertilization the characteristic number of chromosomes in a single individual member of the human species (46) can be maintained — otherwise we would end up with a monster of some sort.
To accurately see why a sperm or an oocyte are considered as only possessing human life, and not as living human beings themselves, one needs to look at the basic scientific facts involved in the processes of gametogenesis and of fertilization. It may help to keep in mind that the products of gametogenesis and fertilization are very different. The products of gametogenesis are mature sex gametes with only 23 instead of 46 chromosomes. The product of fertilization is a living human being with 46 chromosomes. Gametogenesis refers to the maturation of germ cells resulting in gametes. Fertilization refers to the initiation of a new human being.
www.dictionary.com
