Fertilisation & Implantation IGCSE Biology

Fertilisation and Implantation: From Gamete Fusion to Embryo

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

Fertilisation is the fusion of the sperm nucleus with the egg nucleus in the oviduct, forming a zygote. The zygote divides repeatedly by mitosis to form an embryo, which implants into the thick lining of the uterus.

This sequence from gamete fusion to implantation is a core narrative that students must describe precisely. Common errors include placing fertilisation in the uterus, confusing the zygote with the embryo, and omitting the role of mitosis in early development.

The process of fertilisation

During sexual intercourse, semen is deposited in the vagina. Sperm swim through the cervix and uterus into the oviduct (fallopian tube). If an egg has been released from the ovary (ovulation), a sperm may reach it in the oviduct.

The sperm penetrates the egg membrane using enzymes from the acrosome. The nucleus of the sperm fuses with the nucleus of the egg. This fusion of the two haploid nuclei is fertilisation, and it restores the diploid chromosome number. The resulting cell is called a zygote.

From zygote to embryo

The zygote begins to divide by mitosis as it travels along the oviduct toward the uterus. Each division produces genetically identical cells. After several divisions, a ball of cells called an embryo forms.

At this early stage, all cells are undifferentiated. They will later specialise into different cell types. The journey from the oviduct to the uterus takes several days, during which the embryo continues to divide.

Implantation in the uterus

Implantation occurs when the embryo embeds itself into the thick, blood-rich lining of the uterus. The uterus lining has been thickened by the hormone progesterone in preparation for receiving the embryo.

Once implanted, the embryo develops a placenta, which connects it to the mother's blood supply. The placenta allows the exchange of nutrients, oxygen, carbon dioxide and waste products between the mother's blood and the fetal blood without the two blood supplies mixing directly.

The role of the placenta and umbilical cord

The placenta has a large surface area with thin walls and a rich blood supply, enabling efficient exchange. Oxygen and nutrients (such as glucose and amino acids) diffuse from the mother's blood into the fetal blood. Carbon dioxide and urea diffuse from the fetal blood into the mother's blood for removal.

The umbilical cord connects the fetus to the placenta, containing fetal blood vessels. The mother's blood and fetal blood do not mix because mixing could cause immune reactions and differences in blood pressure could damage fetal blood vessels.

Try a focused question (4 marks)

Describe the events that occur from fertilisation to implantation.

Show the answer and marking guidance

The sperm nucleus fuses with the egg nucleus in the oviduct to form a zygote.
The zygote divides by mitosis to form a ball of cells (embryo).
The embryo travels along the oviduct to the uterus.
The embryo implants into the thick, blood-rich lining of the uterus.

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Stating that fertilisation occurs in the uterus rather than the oviduct.
Saying the mother's blood mixes with the fetal blood in the placenta.
Confusing the zygote (single fertilised cell) with the embryo (ball of cells).
Forgetting that cell division after fertilisation is mitosis, not meiosis.