6 Essential Stages of Embryo Development After Fertilisation

Article

Understanding the journey of embryo development is incredibly valuable, especially for couples undergoing IVF. By learning about each stage from fertilisation to implantation you’ll gain clarity on what happens inside the body and lab, helping you feel more informed and reassured throughout your fertility journey.

This article breaks down the development of embryos after fertilisation into six key stages, offering a clear explanation of what happens, when it happens, and why it matters for your chances of pregnancy success.

1. Fertilisation (0–24 Hours): Where Life Begins

The first step in embryo development starts when a single sperm successfully penetrates a mature egg. This union forms a zygote a single celled structure containing genetic material from both parents.

This critical moment, which often happens in a lab during IVF, sets the foundation for new life. The egg and sperm must each contribute a complete set of chromosomes, making this step vital for genetic normality.

2. Zygote Formation (Day 1): The First Division

Once fertilisation is complete, the zygote begins its first division, typically within 24 hours. The single cell splits into two, then four, and continues to multiply. At this stage, the embryo is still within the zona pellucida, a protective shell surrounding the developing cells.

A healthy Day 1 zygote is a good sign of viable development to come.

3. Cleavage Stage (Days 2–3): Rapid Cell Division

During the cleavage stage, the embryo undergoes a series of mitotic divisions, typically reaching 6–8 cells by Day 3. These cells, called blastomeres, divide within the same size boundary, so the embryo remains small but increasingly complex.

Embryologists assess cell symmetry and fragmentation, two important indicators of embryo health.

4. Morula Stage (Day 4): Compact Cell Mass

By Day 4, the embryo becomes a morula a compact ball of about 16 cells. At this stage, the cells begin to differentiate and form tight junctions.

A healthy morula is a sign that the embryo is progressing as expected and may reach the blastocyst stage soon.

5. Blastocyst Stage (Days 5–6): Ready for Implantation

This is a key milestone in embryo development. The blastocyst has three parts:

Inner cell mass: Becomes the fetus
Trophectoderm: Becomes the placenta
Blastocoel: A fluid-filled cavity

A well developed blastocyst has the best potential for implantation. Embryos at this stage may be selected for transfer or frozen for future use.

6. Implantation (Days 6–10): Beginning of Pregnancy

After the blastocyst hatches from its shell, it embeds itself into the uterine lining a process called implantation.

If successful, the embryo begins producing hCG, the hormone detected in pregnancy tests. This is the official start of pregnancy.

Why It Matters

Understanding these developmental milestones empowers patients to engage confidently in their IVF journey. It also sheds light on why some embryos are selected over others for transfer or freezing, and why some may not progress beyond certain stages.