From implantation until the eighth week of pregnancy, your baby is technically referred to as an embryo. A great deal of growth takes place in these eight short weeks. In biological terms, the fertilized oocyte will go through 23 stages of development in 60 days growing from a tenth of a millimeter to about 30 millimeters in length, from a tiny spec into a fetus with arms that bend at the elbow, eyes, hands, feet, ears, fingers, toes and eyelids.
Week 1
The first day of fertilization, the fertilized oocyte measures only .1 to .15 millimeter. On the second and third days, cell division occurs. Cytoplasmic volume reduces as the inner and outer cell mass forms. On the fourth and fifth days, the oocyte is a free blastocyst and loses its zona pellucid. Days five through seven brings about the beginning of the attachment of the blastula to the uterine lining or endometrium.
Week 2
Between days seven and 12, implantation, or the attachment to the uterus, sparks the mother and blastula connection as the umbilical cord is formed. The axis forms at the blastula’s center, the beginnings of spine and spinal cord. Days 13 through 15 brings about the development of the extra-embryonic mesoderm. The blastula measures .4 millimeters long.
Week 3
On days 15 through 17 bring about gastrulation and the notochordal process as the spine develops further. Days 17 through 19 sees the development of the gastrula’s notochordal canal, and days 19 through 21 introduce neural folds, cardiac primordium, and the head fold. The gastrula now measures 2.5 millimeters in length.
Week 4
On days 22 through 26, the neural fold fuses and on the rostral neuropore closes. By the 30th day, the caudal neuropore closes and the embryo now measures between 3 and 5 millimeters long.
Week 5
By now, chemicals are produced that stop the mother’s menses and the embryo’s leg buds, lens placode, and pharyngeal arches develop. Next comes the lens pit and optic cup and then the lens vesicle, nasal pit, and hand plates. By the end of the fifth week, the embryo measures between 7 and 9 millimeters.
Week 6
Days 37 through 44 bring about the development of nasal pits moved ventrally, auricular hillocks, foot plates, and finger rays. Brain wave activity is present as is the heartbeat. Limb buds appear where the arms and legs will grow later. Organogenesis begins. The head represents about one half of the embryo's axial length which is now between 11 and 14 millimeters.
Week 7
Between days 44 and 48, ossification begins and blood type is evident. The trunk straightens between days 48 and 51. Now the embryo measures between 16 and 18 millimeters.
Week 8
Days 51 through 54 bring about the lengthening of the upper limbs and the ability to bend at the elbow. The hands and feet turn inward, eyelids and external ears develop and the head becomes more rounded as the body and limbs are visibly discernable. By the end of the eighth week, the embryo measures between 27 and 31 millimeters long and is officially a fetus.