Every parent is curious to see his or her newborn baby. But before the baby steps out in the beautiful world there are many process take place relating to its development, may it be socially, physically, mentally or emotionally. Thus it becomes very important to know the developments when the baby is in the womb. The child is like a tadpole when just conceived, but nine months later transforms to a complete being.
EMBRYONIC PERIOD
Week 1-2
Gestational age: 0 to 1 (whole) weeks old. 1-14 days from last menstruation.
Embryonic age: -2 to -1 weeks old.
That is, week 1-2 of gestational age are merely theoretical extrapolations of embryonic age, since the fertilization hasn't actually occurred yet.
Week 3
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 zygote.
The zygote undergoes cellular divisions, but does not increase in size. This mitosis is also known as cleavage. A hollow cavity forms marking the blastocyst stage.
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 and performs implantation onto the endometrial lining of the mother's uterus.
If separation into identical twins occurs, 1/3 of the time it will happen before day 5.
Week 4
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 fert.
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.
Week 5
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.
Gastrulation commences.
A neural groove (future spinal cord) forms over the notochord with a brain bulge at one end.
Somites, the divisions of the future vertebra, form. (day 20 of fert.)
Primitive heart tube is forming. Vasculature begins to develop in embryonic disc. (day 20 of fert.)
Week 6
Gestational age: 5 weeks old. 36-42 days from last menstruation.
Embryonic age: Week nr 4. 3 weeks old. 22-28 days from fertilization.
The embryo measures 4 mm (1/8 inch) in length and begins to curve into a C shape.
The heart bulges, further develops, and begins to beat in a regular rhythm.
Structures of the face and neck, form.
The neural tube closes.
The ears begin to form.
Arm buds and a tail are visible.
The first traits of the lung appear.
The first traits of the liver appear.
Buccopharyngeal membrane ruptures. This is the future mouth.
The rectal and urinary passageways become separated.
Spleen appears.
Week 7
Gestational age: 6 weeks old. 43-49 days from last menstruation.
Embryonic age: Week nr 5. 4 weeks old. 29-35 days from fertilization.
The embryo measures 8 mm (1/4 inch) in length
The eye start to develop
Nasal pits form.
The brain divides into 5 vesicles.
Leg buds form and hands form as flat paddles on the arms.
Blood moves through primitive vessels
Week 8
Gestational age: 7 weeks old. 50-56 days from last menstruation.
Embryonic age: Week nr 6. 5 weeks old. 36-42 days from fertilization.
The embryo measures 13 mm (1/2 inch) in length.
Lungs begin to form.
The brain continues to develop.
Arms and legs have lengthened with foot and hand areas distinguishable.
The hands and feet have digits, but may still be webbed.
Week 9
Gestational age: 8 weeks old. 57-63 days from last menstruation.
Embryonic age: Week nr 7. 6 weeks old. 43-49 days from fertilization.
The embryo measures 18 mm (3/4 inch) in length.
Fetal heart tone (the sound of the heart beat)can be heard using doppler
Nipples and hair follicles begin to form.
Location of the elbows and toes are visible.
Spontaneous limb movements may be detected by ultrasound.
All essential organs have at least begun formation.
