Defending the pro-life position sometimes can seem overwhelming and complicated, but it doesn't have to be. A majority of the objections to the pro-life view can be answered with what is called the SLED test. The SLED test states that the unborn differs from the newborn in four ways, none of which are relevant in deciding whether someone has a right to life. The acronym SLED stands for size, level of development, environment, and degree of dependency. This was first developed by Stephen Schwarz in his book The Moral Question of Abortion. This pro-life defense has since been adopted by prominent pro-life writers such as Francis Beckwith, Scott Klusendorf, and Stephen Wagner.
When someone brings up that the unborn is so small that it shouldn't have the same rights as a newborn, then it can be shown that the size of a born person doesn't matter to their value. A lineman in the NFL is obviously larger that a figure skater, but that doesn't make him any more valuable as a person. A toddler is smaller than an adult, but they should be granted the same human rights to live. Thus, the unborn are nearly always smaller than a newborn, but that doesn't mean that they have any less value.
Level of Development
Pro-choicers will often say that the unborn is less developed than a newborn, so they are not fully human. A newborn is less developed than a toddler. A toddler is less developed than an adolescent, and an adolescent is less developed than an adult. Even a mentally handicapped adult might not be as developed as a toddler. In all of these examples we wouldn't think of killing a person because they aren't as developed as someone else. But many believe we should kill the unborn just because they are less developed and can't function as a newborn does.
Of the four parts of the SLED test, environment or location is probably used the most for justifying abortion. Just because people aren't able to see the unborn, then they often think that it doesn't deserve the same human rights as the born child. If you go from one room to another or walk from one house to another, you have the same value. What is unbelievable is that a premature baby can be delivered and given to the mother, but if that baby was just a foot away inside the mother it could be killed. Your geographical location has nothing at all to do with your value as a human being.
Degree of Dependency
A popular argument is that the unborn is less of a human because it is dependent on its mother for life. Through many stages of our lives we are dependent on certain people or machines for our well-being. Some are on kidney machines or pacemakers. The handicapped may be dependent on others just to assist them in routine daily functions. The elderly in nursing homes are dependent on the staff for support. The unborn is relying on the level of dependency from the mother that it is supposed to receive at that stage in life.
It is important to think of human life from conception until death as a continuum. Humans anywhere in that timeline have the same value as other humans, and that isn't affected by their size, level of development, environment, or degree of dependency.