SCIENCE
ABORTION
Ovulation
Ovulation is a key event in the menstrual cycle in which a mature egg is released from the ovary into the fallopian tube, where fertilization may occur. Medical authorities note that ovulation typically occurs near the midpoint of the cycle. As the Mayo Clinic explains, “Ovulation usually occurs about halfway through the menstrual cycle, around day 14 of a 28-day cycle, but timing can vary” (Mayo Clinic 2023).
The process begins when follicle-stimulating hormone (FSH) stimulates the growth of ovarian follicles. According to Guyton and Hall’s Textbook of Medical Physiology, “FSH stimulates the growth of ovarian follicles, though usually only one follicle reaches full maturity” (Guyton and Hall 2021). Rising estrogen levels from the dominant follicle lead to a surge of luteinizing hormone (LH), which triggers ovulation. Speroff and Fritz describe this surge as “the immediate stimulus for ovulation, resulting in rupture of the mature follicle and release of the oocyte” (Speroff and Fritz 2011).
Once released, the egg remains viable for fertilization for a short period. Medical literature states that “the ovulated oocyte remains viable for approximately 12 to 24 hours” (Speroff and Fritz 2011). Following ovulation, progesterone prepares the uterine lining for possible implantation, creating conditions necessary for early human development.
Fertilization
Fertilization is the biological process by which human development begins, occurring when a sperm unites with an egg to form a new, genetically distinct organism. After ejaculation, millions of sperm travel through the female reproductive tract, yet “only a few hundred sperm actually reach the site of fertilization” in the fallopian tube (Moore, Persaud, and Torchia, The Developing Human). When a sperm reaches the egg, it undergoes the acrosome reaction, releasing enzymes that allow it to penetrate the zona pellucida, the egg’s protective outer layer (Sadler, Langman’s Medical Embryology).
Once a single sperm penetrates the egg, the egg completes its final maturation, and the male and female nuclei fuse, “restoring the diploid number of chromosomes” (Moore, Persaud, and Torchia, The Developing Human). This creates a zygote with 46 chromosomes—23 from each parent—forming a unique genetic identity. The zygote then begins rapid cell division while traveling toward the uterus. As embryologist Keith Moore explains, “development begins when a sperm fertilizes an oocyte and together they form a zygote, the beginning of a new human being” (Moore, Persaud, and Torchia, The Developing Human).
A sperm cell fertilizing an egg cell. http://www.pdimages.com/web9.htm, Public domain, via Wikimedia Commons
Fun Fact: DNA Length
The human body carries an extraordinary biological blueprint within every cell. Each human cell contains approximately two meters of DNA, an immense quantity of genetic information compressed into a microscopic nucleus. As Molecular Biology of the Cell explains, “If the DNA from a single human cell were stretched out end to end, it would be about two meters long” (Alberts et al. n.d.).
When considered across the entire body, the scale becomes almost unimaginable. The Human Genome Project has noted that the total DNA contained in all the cells of the human body would extend for “tens of billions of miles if fully uncoiled and laid end to end” (National Human Genome Research Institute n.d.). This distance would be sufficient to span the space between the Earth and the Sun hundreds of times over.
Such staggering biological order reflects not randomness, but remarkable organization and precision. As biologist Francis Collins observed, DNA represents “an instruction book, a software program, that makes us who we are” (Collins 2006). The complexity encoded within every human cell underscores that human life is not disposable or trivial, but profoundly structured, meaningful, and worthy of protection.
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Zygote Development
Conception occurs when a sperm fertilizes an egg, forming a single-celled zygote with a complete and unique genetic identity. As standard embryology texts explain, “fertilization is the process by which the male and female gametes fuse to form a zygote,” creating “a new combination of genes different from that of either parent” (Moore, Persaud, and Torchia 2020, The Developing Human). This zygote represents the earliest biological stage of a new human organism.
Soon after fertilization, the zygote undergoes cleavage, a series of rapid cell divisions in which it “divides repeatedly without increasing in overall size” as it travels toward the uterus (Sadler 2019, Langman’s Medical Embryology). By about five days, it reaches the blastocyst stage, a hollow structure composed of an inner cell mass, which “will give rise to the embryo proper,” and an outer cell layer (Moore, Persaud, and Torchia 2020).
Once in the uterus, the blastocyst begins implantation. The outer layer, known as the trophoblast, “attaches to and invades the endometrium,” initiating placental formation and allowing the developing embryo to receive nutrients and oxygen necessary for continued growth (Sadler 2019; Cleveland Clinic n.d.).
Cell Differentiation
Following implantation, cells within the blastocyst undergo a highly regulated process of cell differentiation, in which previously unspecialized cells begin to assume distinct identities and functions. Developmental biologists describe differentiation as being governed by both intrinsic genetic instructions and extrinsic signaling cues from neighboring cells and the surrounding uterine environment (Gilbert and Barresi, Developmental Biology). This process transforms the early embryo from a relatively uniform group of cells into organized layers with specific developmental destinies.
Early in this stage, the embryo forms three primary germ layers—the ectoderm, mesoderm, and endoderm—a foundational event known as gastrulation. As explained in standard embryology texts, “the three germ layers give rise to all tissues and organs of the body” (Moore, Persaud, and Torchia, The Developing Human). The ectoderm develops into structures such as the skin and nervous system; the mesoderm forms muscles, bones, the heart, and the circulatory system; and the endoderm gives rise to the lining of the digestive and respiratory tracts.
Bone Cell Differentiation
Neuronal Cell Differentiation
Cartilage Differentiation
As development progresses, cells within each germ layer continue to specialize through organogenesis, the process by which distinct organs and body systems are formed. According to the National Institutes of Health, organogenesis establishes “the basic structural plan of the body” as tissues organize into functional organs. During this period, the neural tube, which later becomes the brain and spinal cord, forms from the ectoderm—a critical milestone in early nervous system development (Sadler, Langman’s Medical Embryology).
At the same time, mesodermal cells contribute to the formation of the primitive heart, which begins rhythmic contractions early in embryonic development. Embryology literature notes that this early heartbeat marks the establishment of a functioning circulatory system capable of moving blood through the developing embryo (Moore, Persaud, and Torchia, The Developing Human). Together, these coordinated processes demonstrate the rapid and continuous progression from a single implanted embryo to an organized, developing human body.
Pre-Embryonic Stage (Weeks 1-3)
Following fertilization, the zygote undergoes a series of rapid mitotic divisions known as cleavage while traveling through the uterine tube toward the uterus. During this process, the overall size of the conceptus does not increase; rather, the cells become progressively smaller, forming a compact mass. By approximately day five, this structure develops into a blastocyst, characterized by an inner cell mass and an outer trophoblast layer. As noted in embryological literature, “the blastocyst implants in the endometrium approximately six days after fertilization” (Keith L. Moore, T. V. N. Persaud, and Mark G. Torchia, The Developing Human: Clinically Oriented Embryology).
Implantation marks a critical transition in early development, allowing for direct physiological interaction between the developing human and the maternal environment. The trophoblast begins to invade the uterine lining, initiating nutrient exchange essential for continued growth. Although no external physical features are yet visible, significant biological organization is already underway. Even at this early stage, “cells of the inner cell mass begin to differentiate into distinct cell populations that will give rise to all tissues and organs of the body” (T. W. Sadler, Langman’s Medical Embryology). This early differentiation establishes the foundational architecture for the nervous system, circulatory system, and all other major body systems.
Embryonic Stage (Weeks 3-8)
The embryonic period, spanning approximately weeks 3 through 8 after fertilization, is a phase of rapid development during which the basic structure of the human body is established. According to the National Library of Medicine, this period is defined by “the formation of the major organ systems and the establishment of the basic body plan” (National Library of Medicine n.d.).
During week three, gastrulation forms the three primary germ layers — ectoderm, mesoderm, and endoderm — which give rise to all tissues and organs. At the same time, neurulation begins, initiating the development of the neural tube, the precursor to the brain and spinal cord.
The heart begins forming early in this stage, with medical literature noting that the embryonic heart “begins to beat by approximately day 22 to 23,” or early week four (National Library of Medicine n.d.). Limb buds appear shortly thereafter, around weeks four to five, marking the early development of the arms and legs.
By the end of week eight, the embryo has a clearly recognizable human body plan, with developing facial features, fingers, and toes. As described by Stanford Children’s Health, “most major organs have begun to form” by this point, making the embryonic period one of the most critical stages of human development (Stanford Children’s Health n.d.).
Early Fetal Development (Weeks 9-12)
Beginning in week nine of gestation, the developing human enters the early fetal period, marking a transition from the formation of basic structures to their growth, refinement, and functional maturation. Medical texts describe this stage as one in which nearly all major organ systems are already present and continue to develop in size, complexity, and coordination (Donovan, NCBI Bookshelf).
During weeks 9–12, the heart functions as a coordinated four-chambered organ, actively pumping blood through an increasingly organized circulatory system. Developmental physiology research notes that the transformation from a simple heart tube into a fully partitioned heart capable of effective circulation is largely complete by this point (Männer, Frontiers in Physiology).
The brain and nervous system undergo rapid growth as neural connections expand and become more integrated, laying the groundwork for future sensory and motor function. At the same time, ossification continues, with bone progressively replacing cartilage throughout the skeleton—a process that begins earlier in development and advances steadily during this period (Breeland, NCBI Bookshelf).
The fetus also begins making spontaneous movements, reflecting early neurological activity and muscular coordination. While these movements are not yet perceptible to the mother, clinical sources note that they are a normal and expected part of early fetal development (Cleveland Clinic, “Fetal Development”).
By the end of week twelve, the fetus displays distinctly human features, with proportioned limbs, developing facial characteristics, and functioning organ systems that will continue to mature throughout the remainder of pregnancy.
Second Trimester (Weeks 13-28)
The second trimester is marked by significant advances in sensory perception, motor coordination, and overall physical growth. By approximately week 16, the eyes have formed and the retina is capable of responding to light, even though the eyelids remain fused until roughly weeks 24–26. As described in The Developing Human by Moore, Persaud, and Torchia, “photoreceptors are present in the retina by mid-gestation, allowing light sensitivity before eyelid separation.” (Moore, Persaud, and Torchia 2020)
Hearing also emerges during this period. Research summarized in Fetal and Neonatal Physiology indicates that by around weeks 18–20, the auditory system has developed sufficiently for the fetus to detect and respond to sounds, including low-frequency external noises and the mother’s voice. Studies have demonstrated measurable fetal heart-rate changes in response to sound stimuli, reflecting early auditory processing. (Polin, Fox, and Abman 2017)
The sense of touch becomes increasingly refined as cutaneous sensory receptors mature. According to the National Institutes of Health, tactile responsiveness begins earlier in gestation and becomes more coordinated during the second trimester, enabling the fetus to respond to pressure and movement. Muscular and neurological development also accelerates, producing stronger, more purposeful movements. These movements are often felt by the mother between weeks 18 and 20, a milestone traditionally known as quickening. (National Institutes of Health 2022)
By the later part of the second trimester, organ systems continue maturing in preparation for life outside the womb. The American College of Obstetricians and Gynecologists notes that “viability is possible beginning around 24 weeks of gestation, though survival depends on gestational age and access to advanced neonatal care.” During this time, the lungs undergo critical structural development, including the formation of primitive air sacs, while other organs continue functional refinement essential for postnatal survival. (American College of Obstetricians and Gynecologists 2021)
Third Trimester (Weeks 28-birth)
The third trimester represents the final stage of fetal growth and physiological maturation. The American College of Obstetricians and Gynecologists (ACOG) defines this period as beginning at 28 weeks of gestation and continuing until birth, during which the fetus experiences rapid weight gain and prepares for life outside the womb (American College of Obstetricians and Gynecologists 2020). A key focus of this stage is lung maturation, as the lungs continue producing increasing amounts of surfactant, a substance essential for keeping the air sacs open after birth and enabling effective breathing.
At the same time, the brain undergoes an intense phase of development. Neuroimaging studies demonstrate that cerebral volume increases dramatically during the third trimester, accompanied by the formation of complex folds (gyrification) that support higher neurological function. Sensory systems also continue to mature. By this stage, the eyes are capable of opening and responding to light, and research on fetal and newborn behavior shows that babies can recognize familiar sounds, including the mother’s voice and heartbeat, indicating auditory learning before birth.
Subcutaneous fat accumulates rapidly beneath the skin during the final months of pregnancy, giving the baby a smoother appearance while also playing a crucial role in energy storage and temperature regulation after delivery. By the final weeks of gestation, most organs are fully formed and capable of functioning independently, although continued growth and refinement occur until birth. According to ACOG, infants born during the third trimester, while still premature, often have a strong likelihood of survival with appropriate medical support as their bodies complete the transition toward independent life outside the womb (American College of Obstetricians and Gynecologists 2020).