Prenatal: the time from conception to birth. it is a time of tremendous growth form a single cell to an organism complete with brain and behavioral capabilities, produced in approximately 9 -month period
When semen is deposited in the vagina, the spermatozoa travel through the cervix and body of the uterus and into the Fallopian tubes. Fertilization of the ovum (egg cell) usually takes place in the Fallopian tube. Many sperm must cooperate to penetrate the thick protective shell-like barrier that surrounds the ovum. The first sperm that penetrates fully into the egg donates its genetic material (DNA). The egg then polarizes, repelling any additional ...view middle of the document...
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.
Gestational age: 0 to 1 (whole) weeks old. 1–14 days from last menstruation.
Embryonic age: -2 to -1 weeks old. (Week 1–2 of gestational age are theoretical extrapolations of embryonic age, since the fertilization hasn't actually occurred yet.)
Gestational age: 2 (whole) weeks 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.
Gestational age: 3 weeks 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 happen between days 5 and 9. If it happens after day 9, there is a significant risk of the twins being conjoined.
* Primitive streak develops. (day 13 of fertilization.
* Primary stem villi appear. (day 13 of f fertilization..)
Gestational age: 4 weeks old. 29–35 days from last menstruation.
Embryonic age: Week nr 3. 2 weeks old. 15–21 days from fertilization.
* A notochord forms in the center of the embryonic disk. (day 16 of fertilization.
* Gastrulation commences. (day 16 of fertilization.
* A neural groove (future spinal cord) forms over the notochord with a brain bulge at one end. Neuromeres appear. (day 18 of fertilization.)
* Somites, the divisions of the future vertebra, form. (day 20 of fertilization.
* Primitive heart tube is forming. Vasculature begins to develop in embryonic disc. (day 20 of fertilization....