Once you have established that the unborn are human from fertilization, the next step is to ask when we should assign basic human rights to a human individual . The right to life is the most fundamental of all rights since without it you can’t enjoy any other rights. It’s pretty difficult to enjoy freedom of speech if you’re not alive to speak in the first place.
As a Christian, I believe that all humans are valuable because we were made in God’s image.  God does not have a physical body, so we weren’t made in His physical image. We were made in the image of His likeness; in other words, God has a rational, moral nature, and made us with a similar rational, moral nature.
The pro-life view is that basic human rights should be established when the human comes into existence, that is, at fertilization. In fact, I hesitate to use the term “person” because it’s a legal term that has been used to legally discriminate against groups of people in the past (such as Africans when slavery was legal). So when I use “person” it’s usually synonymous with “entity with basic rights” (e.g. the right to life).
The view held by most pro-life advocates is the Substance View, which has its roots in the sixth century Christian philosopher, Boethius: “a person is an individual substance that has a rational nature.”  A substance is essentially something that maintains its identity through change. You are essentially the same being now as the embryo you were in the womb. You can cut off an arm and still be you. Since you are the same substance, if a morally justifiable reason is needed to kill you now, a morally justifiable reason is needed to kill you in the womb. So if anyone is going to support abortion, a reason must be given that could not also be applied to someone outside the womb, otherwise killing that person outside the womb would also be morally justifiable.
The only truly consistent position is the pro-life position, which holds that the unborn are human from fertilization. Basic human rights should be established as soon as the human comes into existence. By contrast, the pro-choice position establishes basic human rights at a certain arbitrary point in human development.
Furthermore, the pro-life view is the all-inclusive view, whereas the pro-choice view excludes certain humans based on their lack of some arbitrarily-decided-upon feature (or point in their development). But to the pro-life advocate, all humans are valuable based on their inherent capacity as rational, moral agents. The human is both a rational and a moral being. Without a moral nature there would be no true humanity, so those who would abolish the moral law would abolish humanity in the bargain.  As C.S. Lewis writes, “Either we are rational spirit obliged for ever to obey the absolute values of the Tao, or else we are mere nature to be kneaded and cut into new shapes for the pleasure masters who must, by hypothesis, have no motive but their own ‘natural’ impulses. Only the Tao provides a common human law of action which can over-arch rulers and ruled alike. A dogmatic belief in objective value is necessary to the very idea of a rule which is not tyranny or an obedience which is not slavery.” 
It often helps in discussions with pro-choice advocates to make sure you listen carefully and accurately understand what their actual argument is, rather than assuming it. It helps to make a distinction between the humanity of the unborn and their personhood. Sometimes when someone accuses the unborn of not being human, they really mean they don’t believe we should afford them basic human rights, or personhood. If they really mean the unborn isn’t alive or isn’t human, then you can refer to my previous article about how we know the unborn are unique, living, human organisms. But if they mean the unborn are not persons, then the conversation will most likely be led in the following direction.
Most pro-choice objections you will encounter will usually fall under one of four categories, and you can remember these by the acronym SLED, as conceived by philosopher Stephen Schwartz.  SLED stands for Size, Level of Development, Environment, and Degree of Dependency. An objection raised that falls under one of these categories argues that the unborn aren’t human, or aren’t a person. After looking at these objections we’ll analyze a few others which have to do with function and socioeconomic problems. There are other, more difficult objections which I’ll write about in a future post. For now, these are some of the more common objections you’ll encounter.
For each of these objections, it helps to affirm the difference. This establishes common ground with the pro-choice advocate. Yes, the unborn are smaller, less developed, etc. than we are. But then you’ll want to ask why it matters. Finally, point to someone outside the womb who has those same differences and ask if it would be okay to kill them for that same reason. 
Size — the unborn is certainly much smaller than we are, but two-year-old children are much smaller than adults. Women are generally much smaller than men. But does this mean that two-year-old children have less rights than adults, and women have less rights than men because they’re smaller? It would not be unfair for a basketball coach to choose Shaquille O’Neal for his team over Gary Coleman, but it would be equally wrong to kill either one of them.
Level of Development — the unborn are certainly less developed than we are. Two-year-olds are less developed than adults. Does this mean that two-year-olds have less rights as humans than adults do?
Environment — the unborn are in a different place than we are. They’re in the womb. Changing location doesn’t change your nature or your value. I flew to Italy three years ago but who I was didn’t change. So how does an eight-inch journey down the birth canal change one’s value or nature?
Degree of Dependency — the unborn are much more dependent than we are. But how does being more dependent make us less valuable? It seems to me that someone who is more vulnerable deserves that much more protection. Children can’t drive, so they are more dependent than their parents are, who have driver’s licenses. But does it follow that adults may kill their children because they’re more dependent? Some say that the fact that they are totally dependent on one person means that person has the right to kill them. But how does that follow?
First, it seems that only being dependent on one person makes you less of a burden than being dependent on many people. But second, as Justice for All’s Executive Director David Lee says, suppose you’re the last out of a public pool and you hear a splash from the deep end. You look in the water and a toddler has fallen in and is drowning. No one else is there but you. That child is completely dependent on you for its survival — are you morally justified in walking away and letting the child die?
Some say that it’s okay to kill the unborn because they can’t feel pain. I think when someone says this they really mean it’s better to kill someone as an embryo because they won’t be in pain. But still, the lack of feeling pain does not mean it’s morally justified to kill someone, otherwise you would be justified in killing someone in their sleep, or through a painless method.
Take the case of Gabby Gingras, born with congenital insensitivity to pain.  This would mean that it would be morally justifiable to kill someone with this condition for any reason that would be used for a similar abortion.
Some also say consciousness or self-awareness is what establishes value. The problem with self-awareness is that we’re not self-aware until sometime after birth. So this would justify infanticide (and some pro-choice philosophers, such as Michael Tooley and Peter Singer, support infanticide for this very reason). Plus, if the immediately exercisable capacity for consciousness is what establishes value, then we could kill anyone who loses consciousness. This would mean we would be morally justified in killing someone for any reason who falls asleep, enters a reversible coma, or goes under anesthesia before a major surgery.
Additionally, as Francis Beckwith and Patrick Lee note, if consciousness is required to bestow value on a human, then no humans are intrinsically valuable. Consciousness is intrinsically valuable. This would mean that the moral rule would be to maximize valuable states of functions. It would not be morally wrong to kill a child, no matter what age, if doing so enabled one to have two children in the future, and thus to bring it about that there were two vehicles of intrinsic value rather than one. 
The thing about pain, self-awareness, or consciousness (aside from the problems already mentioned) is that these are Level of Development problems. So point to a two-year-old, or another human outside the womb who also fails in that way, and ask if it’s morally justifiable to kill someone just because they’re less developed than we are.
Finally, there are certain objections that rely on socioeconomic problems. For example, they might say that a family can’t afford another child, or that overpopulation is an issue, etc. Someone making these arguments is simply assuming that the unborn aren’t human, so in an argument like this it helps to bring the argument back on topic (to what is the unborn?) by asking if these same reasons could be used to justify killing a two-year-old child. A family of six could not kill their two-year-old child to help feed their other children, so we can’t justify abortion for this reason. We can’t go around killing small children or homeless people to help with overpopulation, so we can’t justify abortion for this reason either. Trotting Out the Toddler is a powerful tool to help keep the discussion on what the actual issue is, the nature of the unborn .
 Note here that as a JFA mentor, we actually take a slightly different approach than the one presented here. The scope of this article is how to defend the position that personhood should be established at fertilization, but in JFA seminars we prefer to keep the focus on what the unborn is. I have used both approaches in my discussions with pro-choice advocates.
 Genesis 1:26
 Ancius Manlius Severinus Boethius, Liber de Persona et Duabus Naturis, ch. 3.
 Lewis, C.S., The Abolition of Man, p. 77.
 ibid., pp. 84-85. Note that when C.S. Lewis speaks of the Tao, he is referring to an objective moral law.
 Schwartz, Stephen D., The Moral Question of Abortion, Chicago: Loyola University Press, 1990, pp. 15-19.
 Credit goes to JFA for this approach to using the SLED tool in a dialogue.
 Note that this article is a little graphic.
 Paraphrased from Francis J. Beckwith, Defending Life: A Moral and Legal Case Against Abortion Choice, (Cambridge University Press: Cambridge, New York, 2007), p.50, and Patrick Lee, Abortion and Unborn Human Life, (Washington, DC: Catholic University of America Press, 1996), p. 55.
 Credit goes to Scott Klusendorf and Greg Koukl for the tool of Trotting Out the Toddler.