After fertilization, the single-celled zygote splits into two, then the two cells double to four, four to eight and so on. The journey along the Fallopian tube is quite slow, while growth continues.
On its way, the zygote divides to make a clump of 32 cells, known as the morula stage. If the early embryo splits into two clumps before this, it may develop into identical twins. Every cell in the morula could still become part of the growing embryo.
By the time the womb cavity is reached, the cell cluster becomes hollow and filled with fluid; it is now referred to as the blastocyst. A blastocyst is an embryo that has developed to the stage where it has two different cell types: the surface cells, or outer coat, will become – among other things -the placenta that nourishes the baby. The inner cells will become the fetus itself. On contact, the blastocyst burrows into the uterine wall for nourishment; this process is known as implantation.
The embryonic stage begins in the fifth week. From weeks five to eight, development is rapid. Major body organs and systems will begin to emerge. At this time, the first bone cells will also appear. By the end of the eighth week, the embryo is known as a fetus and looks like a mini human.
Week by week
Week 1 – Within one week of conception, the fertilized egg, known as a blastocyst, will make its way to the uterus. Within days the cells will arrange themselves into two masses: the outer coat will become the placenta, while the inner cell mass becomes the fetus. All being well, the developing embryo will settle into the folds of the womb lining.
Week 2 – The inner cells of the embryo divide into two layers: the ectoderm and the endoderm. The tissues and organs of the body will eventually develop from these. The amniotic sac, which will soon form a protective bubble around the embryo, also starts to develop. The embryo, now completely embedded in the womb, is a disc-shaped mass of cells, measuring roughly 0.2mm (0.008in) in diameter.
Week 3 – At the start of week 3 a groove will form towards what will become the tail end of the embryo; this is the primitive streak. A new layer of tissue-the mesoderm-will develop from the primitive streak. The spinal cord, kidneys and major tissues will all grow from this. Cells from the ectodermal tissue create the neural fold and plate, the first stages in the development of the nervous system. The neural groove will go on to form the spine.
Week 4 – The kidneys are forming from mesodermal tissue and the mouth is emerging. A basic spinal cord and gut now run from the head to the tail. The head and tail fold downward into a curve as a result of the embryo developing more rapidly from the front. The heart tube bends into a U shape and blood begins to circulate around the body.
Week 5 – Pharyngeal arches that develop in the face, jaws, throat and neck appear between the head and body. A complex network of nerves and blood vessels are developing. The embryo’s eyes have formed and the ears are becoming visible. The spleen and pancreas are beginning to develop in the central part of the gut. The thymus and parathyroid glands develop from the third pharyngeal arch. The arms and legs begin to emerge as paddle-shaped buds.
Week 6 – 42 tissue blocks have formed along the embryo’s back and the development of the backbone, ribs and muscles of the torso begins. The length of the embryo is now 7-8mm (0.3m). The embryo’s heart has established a regular rhythm and the stomach is in place. Ears, nose, fingers and toes are just beginning to appear.
Week 7 – The embryo’s eyelids begin to form from a single membrane that remains fused for several days. At this stage in development, the limb muscles are beginning to form. The chest cavity will be separated from the abdominal cavity by a band of muscles; this will later develop into the diaphragm.
Week 8 – Between the fourth and eighth weeks, the brain has grown so rapidly that the head is extremely large in proportion to the rest of the body. The gonads, or sex glands, will now start to develop into ovaries or testes. The elbows, fingers, knees and toes are really taking shape. Inside the chest cavity, the lungs are developing too. At the end of the eight-week period, the embryo becomes a fetus.