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Fertilized Egg
Begins to exist when the sperm reaches the egg and penetrates into the zona pellucida. -
Start
Fertilization -
12 hours
Pronuclei join together -
Zygote Stage Begins
Occurs when the nuclear membrane of the ovum disappears and each of the 23 chromosomes from the egg and sperm combine making 23 chromosomes. The cell is ready to begin dividing. -
30-36 hours
First round of mitosis {cleavage} -
40 hours
Second round of mitosis -
50-60 hours
Third round of mitosis -
Morula Formed
The morula is the result of the third cleavage. -
Embryo Stage Begins
Occurs when the fertilized egg has divided into the morula, and before it becomes a blastocyst. -
Day 6
64 cell embryo {blastocyst} -
Hatching
The blastocyst hatches from the zona pellucida -
Day 8-10
Implantation -
Day 10
Ectopic pregnancy could occur -
Day 12
Disc with streak on the underside forms mesoderm -
Day 12
hCG begins to be secreted -
Day 15
First missed period -
Day 14-22
Neural tube forms -
Day 21
Heart begins to beat, arm & leg buds appear, yolk sac makes red blood cells -
Day 34, 35, 36
Face plates forming -
Day 35
Cleft lip or palate may occur -
Week 6
Muscles form, bent neck, webbed hands and feet, segmented back -
Fetal Stage Begins
Occurs when all the organs have formed the the genitalia are ready to differentiate. -
Week 7
Fingers & toes separate, eyes & ears formed -
Day 50
Genitalia begins to differentiate, eyelids fuse, amnion surrounds fetus -
Week 9-11
Chorionic villi sampling could be performed -
Week 14-15
Amniocentesis could be performed -
6-7 months
Eyes open, fat added under skin, head out of proportion to the body
