Changes by weeks of gestation Gestational age vs. embryonic age Gestational age is the time that has passed since the onset of the last menstruation, which generally or as standard occurs 2 weeks before the actual fertilization. Embryonic age, in contrast measures the actual age of the embryo or fetus from the time of fertilization. Nevertheless, menstruation has historically been the only means of estimating embryonal/fetal age, and is still the presumed measure if not else specified. However, the actual duration between last menstruation and fertilization may in fact differ from the standard 2 weeks by several days. Thus, the first week of embryonic age is already week three counting with gestational age. Furthermore, the number of the week is one more than the actual age of the embryo/fetus. For example, the embryo is 0 whole weeks old during the 1st week after fertilization. The following table summarizes the various expression systems during week number x of gestation. Week 3 Gestational age: 2 weeks and 0 days until 2 weeks and 6 days old. 15–21 days from last menstruation. Embryonic age: Week nr 1. 0 (whole) weeks old. 1–7 days from fertilization. Fertilization of the ovum to form a new human organism, the human zygote. (day 1 of fertilization) The zygote undergoes mitotic cellular divisions, but does not increase in size. This mitosis is also known as cleavage. A hollow cavity forms marking the blastocyst stage. (day 1.5–3 of fertilization.) The blastocyst contains only a thin rim of trophoblast cells and a clump of cells at one end known as the "embryonic pole" which include embryonic stem cells. The embryo hatches from its protein shell (zona pellucida) and performs implantation onto the endometrial lining of the mother's uterus. (day 5–6 of fertilization.) If separation into identical twins occurs, 1/3 of the time it will happen before day 5. Week 4 Gestational age: 3 weeks and 0 days until 3 weeks and 6 days old. 22–28 days from last menstruation. Embryonic age: Week nr 2. 1 week old. 8–14 days from fertilization. Trophoblast cells surrounding the embryonic cells proliferate and invade deeper into the uterine lining. They will eventually form the placenta and embryonic membranes. The blastocyst is fully implanted day 7–12 of fertilization. Formation of the yolk sac. The embryonic cells flatten into a disk, two cells thick. If separation into identical twins occurs, 2/3 of the time it will