Reproduction, Immunity, and Health
2. Human Development
Learning outcomes
- I can describe the stages of human development from fertilization to birth.
- I can explain how a fertilized egg develops into an embryo and fetus.
- I can identify the functions of the placenta and umbilical cord.
- I can describe major physical and biological changes during human growth and development.
- I can explain how genetics and environmental factors influence development.
Introduction
Human development is a remarkable process that begins with a single fertilized cell and ends with the birth of a fully developed baby approximately 38 weeks after fertilization (or about 40 weeks from the first day of the last menstrual period, the method commonly used in medicine).
During pregnancy, a baby grows from a microscopic zygote into a newborn containing trillions of specialized cells organized into complex organs and body systems.
Understanding human development helps us appreciate how life begins, how the body develops, and why proper nutrition and healthcare are so important during pregnancy.
Stages of Human Development
Human development before birth is divided into several stages.
- Fertilization
- Zygote
- Early Cell Division
- Blastocyst
- Implantation
- Embryo
- Fetus
- Birth
Each stage is marked by important changes in growth and development.
Stage 1: Fertilization
Development begins when:
- A sperm cell joins with an egg cell.
- Their nuclei fuse.
- A zygote is formed.
The zygote contains:
- 46 chromosomes
- Half of its DNA from the mother.
- Half of its DNA from the father.
Fertilization usually occurs in a fallopian tube.
Stage 2: The Zygote
The zygote is the first cell of the new individual.
Although it is only one cell, it already contains the complete genetic instructions needed to develop into a baby.
Soon after fertilization, the zygote begins dividing repeatedly by mitosis.
Stage 3: Early Cell Division
As the zygote travels toward the uterus, it divides into:
- 2 cells
- 4 cells
- 8 cells
- 16 cells
- many more cells
These cells become increasingly specialized as development continues.
After several days, the developing structure is called a blastocyst.
Stage 4: Implantation
About 6–7 days after fertilization, the blastocyst reaches the uterus.
It attaches to the thickened uterine lining in a process called implantation.
The uterine lining supplies nutrients and oxygen while the placenta begins to develop.
Without successful implantation, pregnancy cannot continue.
Stage 5: The Embryo
From about week 2 to week 8, the developing baby is called an embryo.
During this stage:
- The brain begins developing.
- The heart starts beating.
- Arms and legs begin to form.
- Eyes and ears develop.
- Internal organs begin forming.
This is the most important stage of organ development.
Because organs are developing rapidly, the embryo is especially sensitive to harmful substances such as alcohol, tobacco smoke, and some drugs.
Stage 6: The Fetus
From about week 9 until birth, the developing baby is called a fetus.
During this period:
- Bones become stronger.
- Muscles develop.
- Organs continue maturing.
- Body proportions become more balanced.
- The fetus grows rapidly.
By the final weeks of pregnancy, nearly all major organs are fully developed.
The Placenta
The placenta is a temporary organ that develops during pregnancy.
It connects the mother's blood supply with the developing fetus.
Its functions include:
- Delivering oxygen.
- Supplying nutrients.
- Removing carbon dioxide.
- Removing waste products.
- Producing hormones that support pregnancy.
Importantly, the mother's blood and the fetus's blood usually do not mix directly. Materials are exchanged across a thin barrier in the placenta.
The Umbilical Cord
The umbilical cord connects the fetus to the placenta.
It contains blood vessels that:
- Carry oxygen and nutrients to the fetus.
- Return carbon dioxide and wastes to the placenta.
The umbilical cord acts as the fetus's lifeline throughout pregnancy.
Human Growth After Birth
Human development continues long after birth.
Major stages include:
| Stage | Main Changes |
|---|---|
| Infancy | Rapid growth; development of movement and senses. |
| Childhood | Continued growth; language, learning, and coordination improve. |
| Adolescence (Puberty) | Rapid growth, sexual maturity, and emotional development. |
| Adulthood | Full physical maturity and reproductive capability. |
| Older Adulthood | Gradual ageing and changes in body systems. |
Growth and development continue throughout life.
Genetics and Development
Genes inherited from parents influence many characteristics, including:
- Eye colour
- Hair colour
- Height potential
- Blood type
- Many aspects of body structure
Genes provide the instructions that guide growth and development.
Environmental Factors
Development is also influenced by environmental factors.
Examples include:
- Nutrition
- Exercise
- Healthcare
- Disease
- Exposure to harmful chemicals
- Alcohol and smoking during pregnancy
- Access to a safe environment
Healthy environments help children reach their full developmental potential.
Nature and Nurture
Human development depends on both:
- Nature (genetics) — inherited characteristics.
- Nurture (environment) — experiences and surroundings.
Most human characteristics result from an interaction between these two influences.
For example:
A person may inherit the potential to become tall, but poor nutrition during childhood may prevent them from reaching that potential.
Real-World Applications
Understanding human development is important in:
- Medicine
- Obstetrics
- Pediatrics
- Genetics
- Prenatal healthcare
- Nutrition
- Public health
- Child development
Knowledge of human development helps doctors monitor healthy pregnancies and identify potential problems early.
Worked Examples
Example 1
What is the first cell formed after fertilization?
Answer:
The zygote.
Example 2
Where does implantation occur?
Answer:
In the lining of the uterus (endometrium).
Example 3
What is the main function of the placenta?
Answer:
To exchange oxygen, nutrients, and waste products between the mother and the fetus while keeping their blood supplies separate.
Example 4
At what stage is the developing baby called a fetus?
Answer:
From about week 9 until birth.
Example 5
Give one genetic factor and one environmental factor that influence development.
Answer:
- Genetic factor: eye colour or blood type.
- Environmental factor: nutrition, healthcare, or exposure to harmful substances.
Did You Know?
By about 18–20 weeks of pregnancy, many mothers can feel the fetus moving for the first time. These movements, sometimes called "quickening," become stronger as the muscles and nervous system continue to develop.
Key Terms
| Term | Definition |
|---|---|
| Fertilization | The fusion of a sperm cell and an egg cell to form a zygote. |
| Zygote | The single cell formed immediately after fertilization. |
| Blastocyst | A hollow ball of cells that forms before implantation. |
| Implantation | The attachment of the blastocyst to the uterine lining. |
| Embryo | The developing human from approximately weeks 2–8 after fertilization. |
| Fetus | The developing human from about week 9 until birth. |
| Placenta | A temporary organ that exchanges nutrients, oxygen, and waste products between the mother and fetus. |
| Umbilical Cord | The structure that connects the fetus to the placenta. |
| Genetics | The inherited information passed from parents to offspring through genes. |
Key Takeaways
- Human development begins with fertilization, forming a zygote.
- The zygote divides repeatedly, becomes a blastocyst, implants in the uterus, develops into an embryo, and later becomes a fetus.
- The placenta and umbilical cord provide oxygen, nutrients, and waste removal throughout pregnancy.
- Growth continues after birth through infancy, childhood, adolescence, adulthood, and older adulthood.
- Both genetics and environmental factors influence how humans grow and develop.
- Healthy prenatal care, good nutrition, and a safe environment play vital roles in supporting healthy development.