THE BEGINNING OF HUMAN LIFE

During procreative intercourse, millions of sperm travel through the female reproductive tract toward the fallopian tube, where one of the sperm may fertilize an egg released during ovulation. The first sperm to penetrate the zona pellucida—the outer layer of the egg—fuses its 23 chromosomes with the egg’s 23 chromosomes, forming a zygote—a unique and individual human organism with a complete genetic blueprint, including: sex, hair color, eye color, and countless other traits (Tomás 2009) (Cherry 2024). It is therefore a biological and undeniable fact—and a consensus among 96% of biologists—that human life begins at conception when the process of fertilization forms a new human being (Arey 1965) (Moore 2016) (Jacobs 2021).

OVERVIEW OF PRENATAL DEVELOPMENT

1. Fertilization: Fusion of the father's sperm with the mother's egg, marking the beginning of an individual human being's life and existence. The human at this stage is called a zygote. (Lazzara 2020) (Torchia 2024)

2. Cleavage: Within 24-36 hours of fertilization, the zygote begins rapid cellular division through a process called cleavage. Around three days after fertilization, a morula is formed, a multi-celled human that is still traveling through the fallopian tubes. (Cherry 2024)

3. Blastulation: Around five days after fertilization, the morula develops into a blastocyst. A blastocyst is an early-staged embryo that takes on the structure of a hollow ball of cells. The blastocyst will begin moving from the fallopian tubes and into the uterus for implantation. (Cherry 2024)

4. Implantation: Approximately six to ten days after fertilization, the blastocyst reaches the uterus and begins the process of implantation. The blastocyst attaches himself or herself to the uterine wall, where he or she will receive nutrients and begin to establish the placenta. Successful implantation, which is usually complete by the end of the second week, marks the end of the germinal stage and the beginning of the embryonic stage. (Cherry 2024)

5. Embryogenesis: The embryonic stage spans from week 3 to week 8 post-fertilization, during which the embryo undergoes rapid development. In the first month (weeks 3-4), the germ layers (ectoderm, mesoderm, endoderm) form, the neural tube forms, the heart starts beating, and the amniotic sac and placenta take shape. By the second month (weeks 5-8), organogenesis accelerates, limb buds appear, facial features emerge, and the neural tube closes, completing early formation of the brain and spinal cord. By week 8, the embryo measures about 1 inch and has a distinctly human-like form. (Cleveland Clinic 2024)

6. Fetal Development: The fetal stage spans from week 9 to birth, focusing on growth and organ maturation. By weeks 9-12, the fetus grows to 3-4 inches, facial features refine, reproductive organs differentiate, and movement begins, with pain perception now being possible. By weeks 13-20, the fetus reaches 6-8 inches, quickening (first movements) occurs, hearing develops, and survival outside of the womb (viability) becomes possible after around 20 weeks. By weeks 21-28, the lungs and brain mature, sensory responses improve, and the fetus grows to 14-16 inches, 2-3 pounds. By weeks 29-36, organs mature and movements increase. By weeks 37-40+, the fetus is full-term at 19-21 inches, 6-9 pounds, and ready for birth. (Cleveland Clinic 2024)

OVERVIEW OF PRENATAL DEVELOPMENT

When we use words such as zygote, embryo, or fetus, we are simply referring to stages of human development. Calling an unborn baby one of the aforementioned terms does not mean that he or she is not a baby or human, just as making a distinction between toddlers and adults does not mean that toddlers are less valuable or less human than adults. The stage of development we are in does not dictate how much of a human we are; all it tells us is how far along we are in life's journey. An unborn baby (which is a term that encompasses zygotes, embryos, and fetuses) is just as valuable as a newborn baby, and both should be granted equal protection under the eyes of the law.

The stages of human growth and development are:

1. Zygote: The earliest and shortest stage of human life, a zygote is the human being that forms immediately after fertilization. The single-celled zygote undergoes a process called mitosis, where he or she splits to form two cells. These cells continue to divide, doubling repeatedly, which leads to an exponential increase in the number of cells. This series of rapid cell divisions is called cleavage. Within just a few days, the zygote's mass of cells start to differentiate and organize into more complex structures, eventually forming what is known as a blastocyst (the beginning of the embryonic stage) before implanting in the uterine wall.

2. Embryo: Whilst the term embryo can also encompass a zygote, the more specific usage of this term is to describe a human being in the embryonic stage of development, which encompasses the period from implantation, which happens at around 1-2 weeks after conception, to the eighth week of pregnancy. The embryonic stage is characterized by the rapid development of the body’s vital organs and systems, including the brain and heart. The baby's heartbeat can be detected at around 5-6 weeks after conception. Near the end of the embryonic stage, the unborn child will look more similar to a newborn than how he or she looked before this point.

3. Fetus: Starting from the ninth week of pregnancy until birth, the human being is now called a fetus. This stage is marked by remarkable growth and refinement of the bodily structures, and the increasing physical and mental abilities of him or her (such as moving or sensing outside noises). 

4. Infant: Usually after around 9 months of pregnancy, the human being is born. This marks the beginning of the infancy stage, lasting for the first twelve months after birth. This period is full of rapid growth, as the infant learns to crawl and communicate.

5. Toddler: Lasting from age one to three, the toddler years are characterized as a period of significant cognitive, emotional, and social development. 

6. Child: Whilst the word itself can apply to any human from conception until adulthood, it can also describe a specific stage of development between toddlerhood and adolescence. This stage usually lasts from age four to age nine.

7. Adolescent: Starting at around ten years of age and ending at around eighteen to nineteen years of age, adolescence is the transition period between childhood and adulthood.

8. Adult: From the twenties onward (legally, it is often considered as anyone eighteen or older), adulthood is the stage of human development that involves heightened abilities and performance. The human brain is usually fully developed at 25, and all other organs usually reach their maximum potential during adulthood. Adulthood involves personal and professional growth, and often the building of a family.

9. Elder: From about 65-70 and above, the senior years mark the oldest stage of human growth and development. During this period, most individuals retire from their careers. The average life expectancy for humans hovers around 80 years, and decline in both cognitive and physical function is present in this stage as a person grows older.

CHARACTERISTICS OF LIFE

The biological characteristics of life consist of cellular makeup, homeostasis, heredity, metabolism, reproduction, environmental response, and evolution. From the moment of conception, an embryo meets all of these characteristics.

1. Cellular Makeup: The embryo is made up of numerous cells (just like us) that are dividing and multiplying rapidly.

2. Homeostasis: The embryo maintains homeostasis by regulating calcium levels, utilizing nutrients, and maintaining stability to grow. 

3. Heredity: At the moment of conception, the embryo's genetic identity is fully established. The embryo inherits many traits from both its mother and father, yet has a unique genetic identity that is distinct from both of its parents. The baby's eye color, hair color, sex, and other genetic attributes are put into place at the moment of conception.

4. Metabolism: The way an embryo is able to grow is through metabolism. An embryo utilizes glucose and other sources of energy to store and utilize energy and nutrients to grow. 

5. Reproduction: An embryo meets this characteristic through its ability to divide and multiply cells as it grows rapidly in-utero.

6. Environmental Response: This characteristic can also be referred to as external stimuli. From the moment of conception, and embryo is already responding to its environment by adapting to it. As the unborn baby grows, it will adapt the way it develops to be able to survive in the environment it is growing in. As it develops further, the fetus is able to move, respond to sounds, recognize their mother's voice, and feel/respond to pain.

7. Evolution: Like all human beings, an embryo meets this characteristic of life due to the ability of its species to evolve and adapt overtime.