The controversy in medical terms is about whether life begins the instant sperm and egg fuse or after it implants in the uterus.

[1] Keith L. Moore. Before We Are Born: Essentials of Embryology [Philadelphia: Saunders], 2008, page 2.

[2] T.W. Sadler. Langman’s Medical Embryology [Philadelphia: Lippincott Williams & Wilkins], 2006 (10th Edition), page 11.

"There is much confusion and disagreement today about when human life actually begins. The official and public face of the medical profession insists that life begins at implantation — yet every embryology and fetology textbook in use today states that it begins at the instant of fusion between sperm and egg. For example, Essentials of Embryology says, “The zygote, formed by the union of an oocyte and a sperm, is the beginning of a new human being.”[1] And Langman’s Medical Embryology says, “Development begins with fertilization, the process by which the male gamete, the sperm, and the female gamete, the oocyte, unite to give rise to a zygote.”[2]"

A simple rational argument... the unborn baby cannot be considered part of the mother, since it has unique DNA.

The biological definition of life...

A living thing pertains to any organism or a life form that possesses or shows the characteristics of life or being alive. The fundamental characteristics are as follows: having an organized structure, requiring energy, responding to stimuli and adapting to environmental changes, and being capable of reproduction, growth, movement, metabolism, and death.

This is NASA protocol for defining life...

"Living things tend to be complex and highly organized. They have the ability to take in energy from the environment and transform it for growth and reproduction. Organisms tend toward homeostasis: an equilibrium of parameters that define their internal environment. Living creatures respond, and their stimulation fosters a reaction-like motion, recoil, and in advanced forms, learning. Life is reproductive, as some kind of copying is needed for evolution to take hold through a population's mutation and natural selection. To grow and develop, living creatures need foremost to be consumers, since growth includes changing biomass, creating new individuals, and the shedding of waste."

from a scientific standpoint, at the moment of fertilization of the egg by the sperm, a completely new organism is present. The organism is not a part of the mother’s body although he or she is located within the mother’s body. Half of the time, the organism is a boy. The genetic complement of the new human being is unique and different from that of the mother.

The argument that this unique human being present in the mother’s womb from the moment of fertilization is a part of the mother’s body until sometime later in gestation is, from a scientific viewpoint, bizarre. What this argument would imply is that human beings reproduce by a process of budding, which is a process by which a new individual organism forms from a part of the mother’s body. This is the means of reproduction of some species of worms but it is most certainly not a means reproduction by human beings.

This argument used by abortion proponents — that an embryo or fetus is a part of the mother’s body until a certain point of gestation — is scientific nonsense. When the argument is made by a scientist, it represents either scientific incompetence or deliberate deception.

So, do you prefer NASA's definition of life, a medical definition, or a biological definition? They all start once a single complete cell begins to perform the functions that define life.

An unborn baby meets all the criteria for life and cannot be "part of the mother" since the unborn baby's cells are not genetically compatible with the mother.

The weird question is other than for debating abortion, why would you presume life begins at a different point with humans than with any other species on the planet? At some point, that seems more convenient than logical.