Fertilisation and the Stages of Prenatal Development (VCE SSCE Health and Human Development): Revision Notes

Fertilisation and the Stages of Prenatal Development

Introduction

Understanding how pregnancy begins and how a baby develops before birth is crucial for health and human development studies. Currently, approximately one in six Australian couples face fertility challenges, making this knowledge particularly relevant.

The prenatal period refers to the time from conception until birth. This stage of development is remarkable because it represents the most rapid period of growth and change in the entire human lifespan. During these approximately 38 weeks, a single cell transforms into a fully formed baby ready to survive in the outside world.

While physical development is the most obvious change during this period, the foundations for social, emotional and intellectual development also begin at this time. The prenatal stage presents both tremendous opportunities for healthy development and significant risks if problems occur.

Sperm, ova and fertilisation

Understanding sex cells

Most cells in the human body contain a structure called the nucleus, which acts like the control centre of the cell. The nucleus holds genetic material that provides instructions for how cells should function and reproduce throughout life. Different types of cells have different abilities to regenerate (regrow and replace themselves).

Sperm and ova (singular: ovum, sometimes called eggs) are specialised sex cells, also known as gametes. These cells are unique because they each contain only half the genetic information needed to create a new human being.

Sperm production

In males, sperm production begins at puberty and continues throughout adult life. Sperm are produced in the testes at an extraordinary rate—over 12 billion per month. This continuous production ensures that males remain fertile throughout their adult years.

Ova production

The production of ova in females is very different from sperm production. Females are born with all the ova they will ever have—these form in the ovaries before birth. Once a girl reaches puberty, these ova begin to mature and are released in a monthly cycle.

The process of fertilisation

Fertilisation (also called conception) is the moment when pregnancy begins. It occurs when a single sperm successfully penetrates an ovum, and the genetic materials from both parents fuse together.

Here's how fertilisation works step by step:

1. During sexual intercourse, millions of sperm are deposited in the vagina
2. The sperm swim through the cervix and uterus towards the fallopian tubes
3. If an ovum is present in one of the fallopian tubes, many sperm will reach it and compete to enter
4. The sperm release enzymes that break down the outer protective barrier of the ovum
5. Once a single sperm penetrates the ovum's membrane, the ovum immediately changes its outer surface
6. This change blocks all other sperm from entering—this is crucial because if more than one sperm entered, the resulting cell would have incorrect genetic information and could not survive

The zygote

When sperm and ovum successfully combine, they create a single cell called a zygote. This zygote contains:

• 23 chromosomes from the sperm
• 23 chromosomes from the ovum
• A total of 46 chromosomes arranged in 23 pairs

Chromosomes are structures made of DNA that carry genetic information. Within chromosomes are genes—the specific instructions that determine the rate and timing of development, the sex of the child, and physical characteristics. Because the zygote receives genetic material from both parents, the resulting child will display a mix of characteristics from both mother and father.

Germinal stage (0-2 weeks)

The germinal stage represents the first two weeks of prenatal development. It begins at fertilisation and ends when the developing cell mass successfully implants into the wall of the uterus.

Journey to implantation

After fertilisation occurs in the fallopian tube, the newly formed zygote doesn't remain static—it immediately begins an important journey:

1. Cell division begins: The zygote starts dividing within approximately 30 hours of fertilisation. This cell division will continue throughout the person's entire life.
2. Travelling down the fallopian tube: While dividing, the zygote moves down the fallopian tube towards the uterus. This journey takes about three to four days.
3. Morula formation: By about three to four days after fertilisation, when the cell cluster contains approximately 16 cells, it forms a solid ball-like structure called a morula.
4. Blastocyst transformation: At around five days after fertilisation, when there are about 64 cells, the structure transforms into a blastocyst. This is a more complex, thin-walled hollow structure with three distinct parts:
   • An outer cell mass (which will eventually become the placenta)
   • An inner cell mass (which will become the embryo)
   • A fluid-filled cavity in the centre
5. Reaching the uterus: The blastocyst enters the uterus and prepares for implantation.
6. Implantation: About seven days after fertilisation, the blastocyst begins to attach itself to the endometrium (the nutrient-rich lining of the uterine wall). This implantation process takes approximately a week to complete.

From blastocyst to embryo

Once implantation is complete, two important changes occur:

• The developing baby is now referred to as an embryo rather than a blastocyst
• The placenta begins to form from the outer cell mass

Characteristics of the germinal stage

Embryonic stage (3-8 weeks)

The embryonic stage is perhaps the most critical period of prenatal development. It begins when implantation is complete (around week 3) and continues until the end of week 8. During this short five-week period, the foundations for all major body systems are established.

Cell differentiation and organogenesis

The embryonic stage is characterised by two crucial processes:

Cell differentiation is when cells begin to take on specialised roles. Instead of all cells being identical, they start to become different types with specific functions. Some cells become heart cells, others become skin cells, brain cells, bone cells, and so on. This specialisation is essential for creating a functioning human body.

Organogenesis refers to the formation of organs. During the embryonic stage, the basic structures of all major organs and body systems begin to develop, including:

• The circulatory system (heart and blood vessels)
• The nervous system (brain and spinal cord)
• The digestive system (stomach and intestines)
• The respiratory system (lungs)
• The urinary system (kidneys)

Physical development during the embryonic stage

By the end of the embryonic stage (week 8), remarkable changes have occurred:

• The embryo is only about 2-2.5 centimetres in length
• The brain and spinal cord (neural tube) are almost complete in structure (though they will continue to grow in size and complexity for years)
• The circulatory system, powered by a beating heart, is the first organ system to function
• Limbs have developed from small buds into recognisable arms and legs
• Fingers and toes are beginning to form
• Facial features are taking shape
• The embryo begins to look distinctly human, though the head and neck account for about half of the total length
• The brain makes up nearly half of the embryo's body weight
• Bone tissue starts replacing cartilage
• Approximately 90 per cent of the structures found in an adult human can be identified in an eight-week-old embryo

The critical period and teratogens

The embryonic stage is the most vulnerable period of prenatal development. Because all major organs and systems are forming during this time, the developing embryo is extremely sensitive to harmful environmental influences.

Teratogens are thought to interfere with the complex process of forming connections between the brain, spinal cord, muscles, and outer parts of the developing body. For coordinated body systems to work properly, these connections must form correctly during the embryonic stage.

Week-by-week embryonic development

Foetal stage (9-38 weeks)

The foetal stage is the final and longest stage of prenatal development, lasting from week 9 until birth at approximately week 38. Once the ninth week begins, the developing baby is referred to as a foetus rather than an embryo.

At the start of this stage, the foetus measures just 2.5 centimetres in length and weighs about 2 grams. By the end, at full term, the baby will typically measure around 50 centimetres and weigh approximately 3500 grams (3.5 kilograms).

Characteristics of the foetal stage

While rapid growth is the most obvious characteristic of the foetal stage, this period involves much more than just getting bigger. The foetal stage is marked by:

• Maturation of all organs and body systems
• Development of sensory abilities
• Preparation for life outside the uterus
• Increasing coordination and movement
• Development of fat deposits for temperature regulation

The role of the placenta

By 14 weeks of pregnancy, the placenta is fully developed and functioning. The placenta is a remarkable temporary organ that plays multiple vital roles during pregnancy.

Development during the foetal stage

Weeks 9-13: During this early foetal period, all body organs are formed but not all are fully functional yet. The foetus grows to approximately 7 centimetres by week 11. Teeth begin forming in the gums, though they won't emerge until after birth. The eyelids are fused shut over the developing eyes and will remain closed for several more weeks.

Weeks 14-18: The foetus grows to about 14 centimetres in length. Several important developments occur during this period. The tongue develops taste buds, allowing the foetus to sense different flavours in the amniotic fluid. The ears become fully functional, and the foetus can hear muffled sounds from the outside world. The sex of the foetus can now be distinguished through ultrasound examination.

Weeks 19-23: By week 22, the foetus measures approximately 33 centimetres in length. The foetus begins swallowing regularly, though it only takes in amniotic fluid. An important milestone occurs when the eyelids separate into upper and lower lids, and the foetus gains the ability to open and close its eyes.

Weeks 24-28: The foetus now measures around 37 centimetres long and weighs approximately 1 kilogram. Fingernails and toenails begin to grow. The foetus's body has grown more, making it more proportionate to the size of the head, though this process will continue until childhood. A crucial development for survival occurs around week 24 when the foetus begins producing surfactant—a substance that reduces surface tension in the lungs and prevents the small air sacs from collapsing during breathing. Babies born after 24 weeks may survive with intensive medical care, and by 28 weeks, most babies are likely to survive if born prematurely.

Weeks 29-33: During this period, the foetus spends most of its time sleeping. Eyebrows and eyelashes grow. Fat deposits form under the skin—this fat layer will help the baby regulate body temperature after birth. The foetus moves in a strong and coordinated way, and these movements are clearly noticeable to the mother.

Weeks 34-38: In preparation for birth, the foetus typically assumes a head-down position. The lungs develop rapidly during these final weeks, continuing to produce surfactant and preparing for breathing air. By the end of this stage, the foetus measures approximately 50 centimetres in length and is ready for birth.

Movement and sensory development

Throughout the foetal stage, movement increases and becomes more sophisticated. The foetus moves almost all parts of its body, and these movements become more noticeable as size increases. Important reflexes develop and strengthen, including:

• Sucking reflex (essential for feeding after birth)
• Grasping reflex
• Breathing movements (though the lungs are filled with amniotic fluid, not air)

The senses also develop during the foetal stage. Around 25 weeks after fertilisation, the foetus may respond to:

• Light
• Sound
• Touch

These senses become increasingly sensitive as development continues.

Other important developments

Sex organs: Sex organs begin taking shape early in the foetal stage. By around week 15, female foetuses will have produced millions of ova, though this number will decrease before birth. Male foetuses' testes begin producing testosterone.

Bones: Bones, which initially consist mainly of cartilage, begin to harden (ossify) during the foetal stage. This ossification process will continue until the end of puberty.

Teeth: Tooth buds form in the gums during the later part of the foetal stage, though teeth won't erupt through the gums until after birth.

Detailed characteristics of foetal development

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