There are four basic ways that the embryo or fetus is different from a newborn baby.
The first difference is size. Embryos are smaller than fetuses who are smaller (usually) than newborns. The question, then, is this. What does size have to do with rights of person hood? The answer: nothing.
Smaller people are no more or less human than those who are bigger. Embryos and fetuses are smaller than newborns just as newborns are smaller than infants and infants are smaller than toddlers and toddlers are smaller than adolescents and adolescents are smaller than teenagers and teenagers are smaller than adults. Size doesn't matter. It is lawful to kill a fly and not lawful to kill a person, not because the person is bigger, but because the person is human. Humanity is what matters. Trees are generally bigger than people, but it is lawful to cut the branches off trees, but unlawful to cut arms off people. Why? Because humanity, not size, is what determines rights of person-hood. This might seem laughably obvious but there are a large number of people out there who actually justify abortion based simply on the tiny size of the embryo or fetus.
Think about this, it is often true that newborn babies, born prematurely, are smaller than other fetuses who are still inside the womb. Those fetuses still in the womb, may be aborted despite the fact that they are bigger than many premature newborns. To argue that abortion is justified because early embryos and fetuses are so small is a rationale with no logical basis. Size doesn't determine person-hood after birth and it shouldn't determine person-hood before birth.
Level of Development
It is quite true that embryos and fetuses are less developed than a newborn (unless, of course, that newborn was born prematurely). But this, too, is a distinction which has no moral significance. It is a difference of degree, not of kind. Physical and/or intellectual development has nothing to do with determining person-hood outside the womb. It is equally insignificant for determining person-hood inside the womb. Children are generally less developed than adults. Adults with developmental disabilities may be less developed than some children, and those with extraordinary mental capacity are no more human than those with lesser IQs. It is humanity, not brain capacity or arm strength that determines person-hood.
A person, as defined by the dictionary, is nothing more or less than a living human. Anyone who tries to narrow this general definition of person-hood does so in an attempt to eliminate a certain group of people who is either getting in their way or has something they want. Creating self-defined definitions of person-hood that are uniquely crafted to eliminate certain individuals from protection under the law has long been the method of choice for implementing all manner of genocidal atrocities.
A person, simply put, is a human being. This fact should be enough.
The unborn child is exactly where he or she should be to mature for 9 months. At around this time (thought to be determined by the child) the child travels a few inches down the birth canal in to the hands of the midwife.
What is it about this short journey that bestows value on to the child? One minute the child can be killed through abortion, the next minute the child’s death would result in a prison sentence. What changes?
The fact is there is no moral difference between a child in the womb and a child outside the womb. The only difference is geographical.
Degree of Dependency
Is our humanity and therefore our worth dependent on how much we need others? Embryos and fetuses are certainly dependent on their mother for their nutrition. Without the mother’s body (until “viability”) the child will die. Through the umbilical cord the child is supplied with oxygen and nutrients.
But think about this: even when the child is born he is still dependent on his parents for his sustenance. The way in which he receives his nutrition may well change from being through the blood to being through either breast or bottle. In fact the child would have to be several years old until he could really be “viable.”
A kidney patient is dependent on her dialysis machine in order to survive. Does this mean she is not valuable? Someone living with diabetes must take insulin to avoid slipping in to a coma. Are we permitted by law then to kill those who are dependent on drugs to live? We are not because our value is not determined by how much we need others.
Embryos and fetuses are not dependent on their mother to be valuable.