Explanation

Summary of important development changes in the human embryo [table] Time from Fertilisation | Organ Formed ; Week 1 | Fertilisation cleavage starts about 24 hours after fertilisation cleavage to form a blastocyst 4-5 days after fertilisation. More than 100 cells implantataion 6-9 days after fertilisation ; Week 2 | The three primary germ layers (ectoderm, endoderm and mesoderm) develop ; Week 3 | Woman will not have a period. This may be the first sign that she is pregnant. Beginning of the backbone. Neural tube develops, the beginning of the brain and spinal cord (first organs) ; Week 4 | Heart, blood vessels, blood and gut start forming. Umbilical cord developing ; Week 5 | Brain developing, ‘Limb buds’, small swelling which are the beginning of the arms and legs. Heart is a large tube and starts to beat, pumping blood. This can be seen an ultrasound scan ; Week 6 | Eyes and ears start to form ; Week 7 | All major internal organs developing. Face forming. Eyes have some colour. Mouth and tongue develop. Beginning of hand and feet ; Week 12 | Foetus fully formed, with all organs, muscles, bones toes and fingers. Sex organs well developed. Foetus is moving ; Week 20 | Hair beginning to grow including eyebrows and eyelashes. Fingerprints developed. Fingernails and toenails growing. Firm hand grip. Between 16 and 20 weeks baby usually felt moving for first time ; Week 24 | Eyelids open. Legal limit of abortion in most circumstances ; By Week 26 | Has a good chance of survival if born prematurely ; By Week 28 | Baby moving vigorously. Responds to touch and loud noises. Swallowing amniotic fluid and urinating ; By Week 30 | Usually lying head down ready for birth ; 40 Weeks | Birth [/table]