The question is misleading because there are many kinds of embryos – fish embryos, chicken embryos, mouse embryos and so on. Embryo is a generic term to describe an early phase in development when a single fertilized cell divides rapidly to form a sheet of cells. This term is mostly used in reference to vertebrate species which have a spinal column such as mammals, birds, reptiles, amphibians, and fishes.
Of course, if we then rephrase the question and ask when does a human embryo become a human- the point becomes moot. Therefore, it is more accurate to ask when does a human embryo become a human being? The reason I bother to point this out is because it is necessary to recognize that use of this word is not reserved for humans only. Most people think that there are only one kind of embryos that exist or matter – the human kind. It is an important reminder that all vertebrate life begins in a similar fashion from a single fertilized egg, progressing to a ball of cells that organize and mature to form different organs and body parts. The final outcome of what the embryos become is defined by differences in the starting genetic material i.e. bird DNA versus fish DNA. Now that the question is clear, I will try and answer it and will presume that the word embryo here refers to the human species. I will also presume that the question is directed towards the controversy of embryos and their potential for life and as tools for medical research.
There is no direct step of conversion from an embryo to a human being. Human gestation is approximately 36-40 weeks from the point of fertilization. And development in the womb is divided into several stages based on the complexity and viability of the fertilized egg. After fertilization, development begins instantly but it takes a few days for the implantation in the uterus. From this point onwards up to 8 weeks after conception, the fertilized egg is considered to be an embryo. Major organ development (heart, brain, digestive system, limbs) occurs during this time and the recognizable body parts are visible. After 8 weeks, medically, the embryo is considered a fetus until birth. When does this fetus transition to a human being? I can list several different criteria that may help in identifying the time and will address them one by one.
Sensory development such as touch and hearing begin to develop after 8 weeks although each sensory faculty develops at different rates. A fetus will respond to light or sounds quite early but at a very basic level. Vision, a key sensory skill, is not fully developed until much later after birth. Most babies cannot focus beyond 10-12 inches and color vision continues to develop during the first few months after birth. So, not all sensory development is complete before birth, although the basic hardware is in place.
Electrical activity of the brain can be recorded via EEG (electro-encephalogram). But electrical activity that is detected in fetuses are not the same patterns that are found in a fully functional being that has the ability to think, plan and reason. These patterns of electrical activity are generally indicative of basic brain stem functions that will regulate the breathing and some motor functions of the fetus, although a fetus will draw it’s first breath of air only outside the womb.
Along similar lines, there is some data to suggest that fetuses can also feel emotions. What does it mean to feel emotions? A hard question to answer but I will suggest a simple explanation for the purposes of this essay. Emotions can be defined as a physiological response such as an increase in hormones that control breathing, heart rate and prepare the body to respond to some external stimulus. By this definition, fetuses can have the physiological equivalent of emotions but not true emotionality. For instance, a quickening of the the fetal heart rate in response to a loud noise or bright light. After birth they have very basic emotional behavior to signal distress or hunger and slowly develop different emotional responses that such as anger, fear or happiness. Higher cognitive abilities of reasoning, morality, language continue to develop much later. Indeed, intellectual and emotional maturation and development can go on endlessly.
Thus far, I’ve listed several criteria that could partially describe a human being. I have left out the ability to reproduce because reproductive maturity is not reached until puberty and is not necessary to be defined as a human being. All of the criteria I have listed suggest are present to some degree in the fetus but only in a nascent, undeveloped form. These faculties continue to develop during the early stages of growth after birth.
So, when does the fetus become a human being?
A fetus is a developing offspring and cannot survive independently outside the womb until 23-28 weeks of gestational age. We know that a fetus born after 23-28 weeks may survive, although development of lungs is not complete at this time and medical assistance to facilitate breathing improves the chances of survival. Of course, once born the infant continues to depend on the mother for nourishment and physical protection. But it can survive. This is the time when a fetus can be deemed a human being, although one that will need nurturing and protection for a long time to fully achieve it’s human potential.
