-
Day 1-6
On day 1 the egg is fertilized and a human zygote is formed. By day three the zygote undergoes cleavage and the blastocyst stage begins. The blastocyst contains a clump of trophoblast cells that form an embryonic pole. Between days 5 and 6 the embryo implants itself onto the mother's uterus. -
Days 7-13
Between days 7-12 trophoblast cells surrounding the embryonic cells invade deeper into the uterine lining. Eventually, the trophoblasts form the placenta and embryonic membranes. On day 13 embryonic cells flatten into a disc, the primitive streak develops, and the primary stem villi appear. -
Days 14-21
By day 18, a notochord forms in the center of the embryonic disc, gastrulation commences, and a neural groove forms over the notochord with a brain bulge. -
Days 22-28
The embryo begins to form a c shape and is about 4mm long. The heart is beginning to beat. Arms, ears, lungs, liver, mouth, pancreas, rectum, spleen, and spinal cord all start to develop. -
Days 29-35
The embryo is 8mm long. Eyes, nose, kidneys, legs, and stomach all start to form. In addition, blood starts moving through the primitive vessels and the brain divides into 5 vesicles. -
Days 36-49
The embryo grows to 18 mm. Limbs lengthen and start to move. The lymphatic system begins to develop. All essential organs have started growing. Finally, the fetal heart tone can be detected. -
Days 50-56
Between days 49-56, the head comprises half the fetus' size. Facial features continue to develop and the eyelids close till later in the pregnancy. Genitals appear and intestines rotate.
