Embryonic Period

The embryonic period is the first 8 weeks of foetal life. It involves the development of the foetus from fertilisation, including key events such as implantation, gastrulation, neurulation, and the formation of all internal organs. This time period transforms the embryo from a cluster of cells to a recognisable human being. This page provides a summary of what happens each week of development, and contains links to other pages for more detailed information on complex events.

Week 1

The first day of human life involves the process of the sperm and egg fusing to start the beginnings of a new life – a process called fertilisation. Once completed, the newly formed zygote drifts towards the uterus whilst undergoing cleavage. By the time the zygote reaches the uterus it has divided into more than 100 cells and is called a blastocyst. The blastocyst reaches the endometrium on approximately Day 6 and begins the process of implantation.

Week 1.1Week 1.2Week 1.3Week 1.4Week 1.5

A = day of fertilisation, B = 1 day after fertilisation, C = 2 days after fertilisation, D = 3 days after fertilisation, E = 4 days after fertilisation, F = 5 days after fertilisation

An animation of the events occurring during the first week of development, namely cleavage and blastocyst formation

Week 2

The second week involves the progression and completion of implantation, which generally happens by around Day 10. As the blastocyst embeds itself into the endometrial lining the outer ball of cells - the trophoblast - differentiates into an outer layer called the syncytiotrophoblast which invades the lining for anchorage and nutrition and secretes human chorionic gonadotrophin (hCG), and an inner layer called the cytotrophoblast. Meanwhile, the bilaminar embryonic disc forms from the inner ball of cells of the blastocyst (embryoblast), and a new cavity – the amniotic cavity – grows deep to the disc. The exocoelomic cavity on the other side of the disc becomes the primary umbilical vesicle, and extraembryonic mesoderm forms between the cytotrophoblast and two cavities. Towards the end of the week lacunar networks form in the syncytiotrophoblast and fill with maternal blood, providing a source of nutrition from eroded uterine glands for the developing embryo, and primary chorionic villi made from trophoblast begin forming on the cytotrophoblast. Fluid-filled spaces form inside the extraembryonic mesoderm, which expand to separate the inner half of the mesoderm from the outer. This forms a cavity surrounding the amniotic cavity and primary umbilical vesicle, called the chorionic sac. The embryo is suspended in this cavity by a connecting stalk.

Week 2.1Week 2.2Week 2.3

A = 8 days after fertilisation

B = 9 days after fertilisation

A = 10 days after fertilisation

B = 12 days after fertilisation

A = 13 days after fertilisation

B = 14 days after fertilisation

Week 3

The embryo rapidly grows during this week and starts to develop some basic structures. Firstly gastrulation occurs in which the embryonic disc differentiates into the three germ layers: ectoderm, mesoderm and endoderm. A primitive streak forms along the disc and cells reorganise themselves. The notochord forms within the mesoderm and serves as a framework for the future vertebral column. The neural plate forms from the anterior ectoderm and sinks into the mesoderm as the neural tube in a process called neurulation. This eventually becomes the central nervous system. Bodies of nervous tissue called somites begin to develop along the length of the disc, starting from the middle with more appearing over time in both directions. These form the axial skeleton and related musculature. A horseshoe space called the intraembryonic coelom appears within the mesoderm which becomes the future body cavities. The primary chorionic villi mature into secondary and then tertiary chorionic villi, and by the end of the week are filled with foetal vessels so transport of nutrients across the placenta can occur. The embryonic cardiovascular system begins to form and the heart starts beating on the last day of the week (Day 21) – this is covered in more detail in the organ development page.

Day 16Day 21

A = 16 days after fertilisation, B = 21 days after fertilisation

Week 4 – 5

In the fourth week the embryo begins to fold. Firstly the head and tail ends fold, transforming the flat disc into the more familiar curved foetal shape. Secondly the sides of the embryo start folding inwards producing lateral folds. Pharyngeal arches begin to develop which form the future pharynx. The rostral neuropore closes sealing the lower end of the neural tube and the brain begins to develop. Towards the end of the week upper and lower limb buds appear (future limbs), and otic pits appear (future eyes).

Day 22Day 23

A/B = 22 days after fertilisation, C/D = 23 days after fertilisation

Day 24Day 26

A/B = 24 days after fertilisation
A/B = 26 days after fertilisation

In the fifth week the caudal neuropore closes sealing the upper end of the neural tube, and the embryo continues to grow – the head enlarges more rapidly than other embryonic structures to become disproportional to the rest of the body.

Day 28Day 32

A/B = 28 days after fertilisation
A/B = 32 days after fertilisation

Week 5

A 5-week old foetus

An animation of the events occurring during the fourth and fifth weeks of development, namely folding of the embryo

Week 6 – 8

In the sixth week the limbs and hands develop and auricular hillocks form which are the primordial external ears. The head bends over the heart, which has formed a large prominence, due to folding. The intestines have been developing and during this week they herniate into the umbilical cord due to the lack of space to grow in – this is normal and is termed umbilical herniation.

Day 42

A/B = 42 days after fertilisation

In the seventh week the limbs develop further and the fingers begin to separate. Apart from general growth not much else happens during this week.
In the eighth week the embryo begins to make movements. The process of ossification of the long bones starts occurring, starting in the femur. The caudal eminence, the tail, has shrunk and almost disappeared. Other features begin to refine such as the shape of the ears and eyes, and the fingers and toes of the limbs have fully separated.

Day 56

A/B = 56 days after fertilisation