When Does Life Begin?

The medical science community
recognizes life begins at conception

"Human development is a continuous process that begins when an ovum from a female is fertilized by a sperm from a male. Cell division, growth, and differentiation transform the fertilized ovum, a specialized cell called a zygote*, into a multicellular adult human being. Most developmental changes occur during the embryonic and fetal periods, but important changes also occur during the other periods of development: infancy, childhood, adolescence, and adulthood.

"Although it is customary to divide development into prenatal and postnatal periods, it is important to realize that birth is merely a dramatic event during development resulting in a change in environment. 
"Development does not stop at birth."

The Developing Human: Clinically Oriented Embryology Fourth Edition, a medical school textbook on embryology by Dr. Keith L. Moore. 
(Dr. Moore is an anatomy professor and associate dean of basic sciences at the University of Toronto Faculty of Medicine. He is former head of anatomy at the University of Manitoba and chairman of the anatomy department at the University of Toronto.) 

*Later in the same passage the term zygote is defined: "A zygote is the beginning of a new human being." 

Other terms follow as defined in
The Developing Human:

Embryo. This term refers to the developing human during the early stages of development.... The embryonic period extends until the end of the eighth week, by which time the beginnings of all major structures are present.

Fetus. After the embryonic period, the developing human is called a fetus. During the fetal period (ninth week until birth), many systems develop further. Though developmental changes are not so dramatic as those occurring during the embryonic period, they are very important.

Other quotes from the book, used to train obstetricians to be:
"In addition to caring for the mother, obstetricians must guard the health of the embryo and the fetus." (7)
"Human development begins at fertilization, when a male gamete or sperm unites with a female gamete or ovum to form a single cell called a zygote (Gr. zygotos, yoked together). This cell marked the beginnings of each of us as a unique individual." (13)

More on the beginning of life
"Since the old ethic has not yet been fully displaced, it has been necessary to separate the idea of abortion from the idea of killing, which continues to be socially abhorrent. The result has been a curious avoidance of the scientific fact, which everyone really knows, that human life begins at conception and is continuous whether intra- or extra-uterine until death. The very considerable semantic gymnastics which are required to rationalize abortion as anything but taking a human life would be ludicrous if they were not so often put forth under socially impeccable auspices."

"A New Ethic for Medicine and Society" California Medicine: The Western Journal of Medicine, 113, no. 3, (1970), pp. 67-68.