How do humans reproduce, and what happens from fertilisation to birth?
Describe sexual reproduction in humans, including fertilisation and the development of the baby in the uterus
A scaffolded answer to the N(A)-Level Biology outcome on human reproduction. The male and female sex cells, what fertilisation is, and how the fertilised egg grows into a baby in the uterus.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
This outcome wants you to describe sexual reproduction in humans: the male and female sex cells, how the sperm is adapted to reach the egg, what fertilisation is, and how the fertilised egg grows into a baby in the uterus. The marks reward a clear definition of fertilisation, linking the sperm and egg to their jobs, and a correct, ordered account of development. Keep the language simple and accurate.
The answer
The sex cells
Sexual reproduction needs two special sex cells, called gametes:
- The sperm is the male sex cell. It is small and can swim, using a tail powered by energy from many mitochondria.
- The egg (ovum) is the female sex cell. It is much larger and contains a food store for the early development of the new life.
Each sex cell carries half the genetic information needed, so when they join the new cell has a full set.
Fertilisation
Fertilisation is the joining (fusion) of the nucleus of a sperm with the nucleus of an egg, forming a single new cell called a zygote. In humans this normally happens inside the mother's body (internal fertilisation), in the oviduct (the tube leading to the uterus).
From zygote to baby
After fertilisation, the events are:
- The zygote divides again and again to form a ball of cells.
- The ball travels to the uterus and sinks into (implants in) its thick lining.
- It grows into an embryo and then a fetus.
- A placenta forms, letting food and oxygen pass from the mother's blood to the baby, and letting waste pass the other way.
- The baby grows in the uterus, cushioned by fluid, until it is ready to be born.
The role of the placenta
The placenta is where the mother's blood comes close to the baby's blood (without mixing). Food and oxygen diffuse from the mother to the baby, and waste such as carbon dioxide diffuses from the baby to the mother. This is how the growing baby is fed and supplied with oxygen before it can breathe or eat for itself.
Examples in context
Example 1. Why a pregnant woman is advised to eat and breathe well. Because the baby gets its food and oxygen from the mother's blood through the placenta, the mother's diet and health directly affect the baby's growth. This is why good nutrition and avoiding harmful substances in pregnancy matter so much.
Example 2. Why twins can be identical or non-identical. Identical twins form when one fertilised egg splits into two, so they share the same genetic information. Non-identical twins form when two eggs are fertilised by two sperm, so they are as similar as ordinary siblings. It shows how fertilisation and early division shape the new life.
Try this
Q1. Name the male and female sex cells in humans. [1 mark]
- Cue. The sperm (male) and the egg or ovum (female).
Q2. Define fertilisation. [2 marks]
- Cue. The fusion of the nucleus of a sperm with the nucleus of an egg to form a zygote.
Q3. Explain how the placenta helps the developing baby. [2 marks]
- Cue. It lets food and oxygen pass from the mother's blood to the baby and lets waste pass back, so the baby is fed and supplied with oxygen.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksDefine fertilisation. Then describe two ways the sperm cell is adapted to reach and fertilise the egg.Show worked answer →
Fertilisation is the joining (fusion) of the nucleus of a male sex cell (sperm) with the nucleus of a female sex cell (egg) to form a single cell called a zygote.
Adaptation 1: the sperm has a tail, which it uses to swim towards the egg.
Adaptation 2: the sperm has many mitochondria in its middle section, which release energy by respiration to power the tail for swimming. (Accept also: a head with enzymes to break into the egg.)
What markers reward: the definition as the fusion of the male and female sex-cell nuclei to form a zygote, and two adaptations linked to the sperm's job of reaching and entering the egg (tail to swim, mitochondria for energy). Saying the sperm carries the baby is wrong.
Original4 marksDescribe what happens from fertilisation to the early development of the baby inside the mother.Show worked answer →
After fertilisation, the zygote divides again and again to form a ball of cells. This ball travels to the uterus and sinks into (implants in) the thick lining of the uterus wall.
There it continues to grow into an embryo and then a fetus. It develops a placenta, which lets food and oxygen pass from the mother's blood to the baby and lets waste pass back. The baby grows in the uterus, protected by fluid, until it is ready to be born.
What markers reward: the zygote dividing to form a ball of cells, implanting in the uterus lining, growing into an embryo/fetus, and the placenta supplying food and oxygen and removing waste. Saying the baby grows in the ovary is a common error.
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