Life begins at conception
The issue of whether life begins at conception is a controversial topic with scientists and religious adherents taking a variety of views. This question is often seen as significant because most moral systems consider human life to be worth protecting and because of its relevance to abortion.
Christian apologists use the common definition of conception, which is at fertilization. Confusingly, the term may also refer to implantation.  This article uses the former definition unless otherwise specified.
Zygote meets the definition of life
The zygote is a single cell that is formed by fertilization/conception and contains all the genetic information needed to form an individual. When the zygote begins to reproduce by division, it is then an embryo. Life is often defined in terms of seven criteria, which the zygote arguably fulfils. 
- "By all the criteria of modern molecular biology, life is present from the moment of conception. "
- "According to Biology, each in every single sign of life is present in the Fetus. "
- "a single-celled organism is viewed as "alive" "
- "the magic biology pro abortion pretends to teach us that the fetus is not a human being but a lifeform X, and from one day to another (seven months + 1 day in some countries for example) ¡¡¡Is a human being!!! WOW thats magic... "
This commits the continuum fallacy.
Scriptural arguments and religious views
The most common Christian view is that life begins at conception, and that since the embryo is alive, the commandment Thou shalt not kill would apply.
- "Human life must be respected and protected absolutely from the moment of conception. From the first moment of his existence, a human being must be recognized as having the rights of a person - among which is the inviolable right of every innocent being to life. "
Christians claim Biblical support for this,  but the cited verses are surprisingly weak.
"Before I formed thee in the belly I knew thee; and before thou camest forth out of the womb I sanctified thee"
"For thou didst form my inward parts: Thou didst cover me [Or, knit me together] in my mother’s womb. [...] My frame was not hidden from thee, When I was made in secret, And curiously wrought in the lowest parts of the earth. Thine eyes did see mine unformed substance; And in thy book they were all written, Even the days that were ordained for me, When as yet there was none of them."
These verses only imply that life exists before birth, not that it starts at conception.
Note that up until the late 1960's, most evangelical Christian leaders were pro-choice!  Their recent attempts to prevent all abortion is a modern innovation.
Life means it can survive independently
- "NO... it begins when the fetus is viable (around 6 months), meaning it can survive without the host (mother), but... Until then, it is a parasite. "
The main counter argument to this is a new born baby cannot survive independently but is clearly alive.
- "If a newborn was left out on the street, it would die."
From developmental features
- "When does a human die? When his heart stops beating. So surely life begins when the heart STARTS beating. This is at roughly 8 weeks."
Although it seems arbitrary to define life beginning in terms of stages of pregnancy, such as when a heart beat is detectable, it is just as arbitrary as choosing conception as the beginning of life.
From the start of pregnancy
Some argue that life begins at the start of pregnancy i.e. at implantation. 
- "However, on the separate but closely related question of when a woman is considered pregnant, the medical community has long been clear: Pregnancy is established when a fertilized egg has been implanted in the wall of a woman's uterus. "
Conception is a process
- "The process of conception, also known as fertilisation, involves many chemical reactions and processes. It is not an instantaneous occurrence. "
If conception is a process, the coming into existence of life also occurs in stages. This may not be compatible with some forms of religious theology.
Life cannot begin until twins are ruled out
A life is considered to be an individual instance or person. However, an embryo can still divide into twins, which seem to be two separate people. So either life can split into two lives, or twins are the same person, or the embryo is not alive. An embryo can split to form twins (or more) up a few weeks after conception.
- "Consider the case of monozygotic twins, as explained by the above diagram. Here we have one fertilisation event, but two individuals result. Do those twins have to share the ‘human life’ they had from conception? Surely not, for we treat twins as separate persons. "
Certain rare humans were formed by the merger of two embyros, which makes them a chimera.  Does this person have two lives? Or can two living things merge to form one life? Alternatively, the embyro is not alive.
Naturally occurring massive death toll
Life beginning at conception leads to the absurd conclusion of a massive, naturally occurring death toll.
- "the results of most conceptions are not viable embryos, and these abnormal embryos are usually passed out during a menstrual cycle. "
"See, the really hard core people will say that life begins at fertilization. Fertilization, when the sperm fertilizes the egg... But, even after the egg is fertilized, it's still six or seven days before it reaches the uteris before pregnancy begins and not every egg makes it that far. 80% of a woman's fertilized eggs are rinsed and flushed out of her body, once a month during [a period]. They wind up on sanitary napkins and yet they are fertilized eggs. So basically what these anti-abortion people are telling us is that any woman who's had more than one period is a serial killer"
- — George Carlin, Pro-Life is Anti-Woman
Some people consider the issue to be more about personhood than life:
- "Human life does certainly begin at conception, but human life isn't particularly valuable. Rights come from personhood, and even then, the expression of personhood is necessary to establish the existence of it at all. "
Another view is that what constitutes "life" is mainly semantics and should not have bearing on our attitude towards abortion. The words "life" and "person" are subjective labels that humans subjectively apply to situations and do not provide any solid basis for morality. The whole debate on when life begins is therefore moot.
- "The truth is that there is no correct answer, each person must form their own ethical and moral conclusion."
Sperm and ovum are alive before conception
Biologically speaking, life is "A distinctive characteristic of a living organism from dead organism or non-living thing, as specifically distinguished by the capacity to grow, metabolize, respond (to stimuli), adapt, and reproduce"  Sperm and ovum are alive before conception, so live does not begin at conception, since it was already there! Although sperm and the ovum cannot survive for long outside the body, neither can the zygote.
Scriptural and religious views
Contrary to claims of Christians,  the Bible does not state that life begins at conception. These Christian teachings are based on a very loose interpretation of poetic language in the Bible. Some verses imply that a person "exists" before they were conceived, which is absurd, but this still does not necessarily imply the soul has entered the body at conception. Some argue that the Bible states life begins with breath: 
"And the Lord God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul."
The Bible also associates life with blood:
- "The earliest blood cells and cardiovascular structures develop during the third week of development. Does this mean that the human being’s life does not begin until blood cells are present? "
"For the life of a creature is in the blood"
Although St. Augustine opposed abortion, he also thought that the soul was not present at conception:
"The law does not provide that the act abortion pertains to homicide, for there cannot yet be said to be a live soul in a body that lacks sensation"
If the issue was that important to God, it would be clearly stated in the Bible. Since this is not the case, it is not important.
In Islam, the classical interpretation is life begins at four months after conception for the purposes of abortion.
- "Abortion is not allowed after four months have passed since conception because at that time it is akin to taking a life, an act that entails penalty in this world and in the Hereafter. "