Week 1: Fertilisation
Week 2: The three basic layers of the embryo develop, namely ectoderm, mesoderm and endoderm.
Week 3: Beginnings of the backbone. Neural tube develops, the beginning of the brain and spinal cord. Embryo about 2mm long.
Week 4: Heart, blood vessels, and blood start forming. Umbilical cord developing. Embryo about 5mm long.
The lip and primary palate develop between the 4th and 7th weeks of gestation.
Week 5: Brain developing. ‘Limb buds’, small swellings which are the beginnings of the arms and legs. Heart starts to beat, pump blood. Embryo about 8mm long.
Week 6: Eyes and ears start to form.
Week 7: All major internal organs forming. Face forming. Eyes have some colour. Mouth and tongue. Beginnings of hands and feet. Fetus is 17mm long.
The secondary palate develops between the 6th and 9th weeks of gestation.
By week 12: Fetus fully formed and moving. Is now 56mm long from head to bottom.
By 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 the first time. Baby is 160mm long from head to bottom.
Week 24: Eyelids open.
By week 28: Baby moving vigorously. Responds to touch and loud noise. Swallowing amniotic fluid and urinating.
By week 30: Usually lying head down ready for birth. Baby is 240mm from head to bottom.
Week 40: Birth
Taylor, D. J. et al. Biological Science 2: Systems, Maintenance and Change. Cambridge UP, 3rd ed., 1997. ISBN 0521567203 (pbk)